docs: enlarge the title used by the ios-build-instructions page

docs/ios_build_instructions.md

-### Build Instructions (iOS)
+# iOS Build Instructions
 
 **Note:** Upstreaming of iOS code is still a work in progress. In particular,
 note that **it is not currently possible to build an actual Chromium app.**
 Currently, the buildable binaries are ios\_web\_shell (a minimal wrapper around
 the web layer), and various unit tests.
 
-Prerequisites
--------------
+## Prerequisites
 
 -   A Mac with a version of OS X capable of running the latest version
     of Xcode.
 -   The latest version of [Xcode](https://developer.apple.com/xcode/),
     including the current iOS SDK.
 -   The current version of the JDK (required for the closure compiler).
 -   [depot\_tools](http://dev.chromium.org/developers/how-tos/install-depot-tools).
 
-Setting Up
-----------
+## Setting Up
 
 In the directory where you are going to check out the code, create a
 `chromium.gyp_env` to set the build to use iOS targets (and to use
 hybrid builds; see [Building](#Building) below):
 
 ```shell
 cat > chromium.gyp_env <<EOF
 {
   "GYP_DEFINES": "OS=ios",
   "GYP_GENERATORS": "ninja,xcode-ninja",
 }
 EOF
 ```
 
 If you aren't set up to sign iOS build products via a developer account,
 you should instead use:
 
 ```shell
 cat > chromium.gyp_env <<EOF
 {
   "GYP_DEFINES": "OS=ios chromium_ios_signing=0",
   "GYP_GENERATORS": "ninja,xcode-ninja",
 }
 EOF
 ```
 
 Also, you should [install API
 keys](https://www.chromium.org/developers/how-tos/api-keys).
 
-Getting the Code
-----------------
+## Getting the Code
 
 Next, [check out the
 code](https://www.chromium.org/developers/how-tos/get-the-code), with:
 
 ```shell
 fetch ios
 ```
 
-Building
---------
+## Building
 
 Build the target you are interested in. The instructions above select
 the ninja/Xcode hybrid mode, which uses ninja to do the actual build,
 but provides a wrapper Xcode project that can be used to build targets
 and navigate the source. (The Xcode project just shells out to ninja to
 do the builds, so you can't actually inspect/change target-level
 settings from within Xcode; this mode avoids generating a large tree of
 Xcode projects, which leads to performance issues in Xcode). To build
 with ninja (simulator and device, respectively):
 
 ```shell
 ninja -C out/Debug-iphonesimulator All
 ninja -C out/Debug-iphoneos All
 ```
 
 To build with Xcode, open `build/all.ninja.xcworkspace`, and choose the
 target you want to build.
 
 You should always be able to build All, since targets are added there for iOS
 only when they compile.
 
-Running
--------
+## Running
 
 Any target that is built and runs on the bots (see [below](#Troubleshooting))
 should run successfully in a local build. As of the time of writing, this is
 only ios\_web\_shell and unit test targets—see the note at the top of this
 page. Check the bots periodically for updates; more targets (new components)
 will come on line over time.
 
 To run in the simulator from the command line, you can use `iossim`. For
 example, to run a debug build of ios\_web\_shell:
 
 ```shell
 out/Debug-iphonesimulator/iossim out/Debug-iphonesimulator/ios_web_shell.app
 ```
 
-Converting an existing Mac checkout into an iOS checkout
---------------------------------------------------------
+## Converting an existing Mac checkout into an iOS checkout
 
 If you want to convert your Mac checkout into an iOS checkout, you can
 follow those steps:
 
 1. Add `target_os = [ "ios" ]` to the bottom of your `chromium/.gclient`
 file.
 
 2. Make sure you have the following in your `chromium/chromium.gyp_env`
 file (removing the `chromium_ios_signing=0` if you want to make
 developer-signed builds):
 
 ```json
 {
   "GYP_DEFINES" : "OS=ios chromium_ios_signing=0",
   "GYP_GENERATORS" : "ninja,xcode-ninja",
 }
 ```
 
 Then make sure you sync again to get all the new files like the following. At
 the end it will run `build/gyp_chromium` which will regenerate all the build
 files according to the new settings.
 
 ```shell
 gclient sync
 ```
 
-Troubleshooting
----------------
+## Troubleshooting
 
 If your build fails, check the iOS columns of [the Mac
 waterfall](http://build.chromium.org/p/chromium.mac/console) (the last
 two) to see if the bots are green. In general they should be, since
 failures on those bots will close the tree.