docs: try to give move precise commands in Xcode-Ninja Hybrid section

docs/mac_build_instructions.md

Index: docs/mac_build_instructions.md
diff --git a/docs/mac_build_instructions.md b/docs/mac_build_instructions.md
index b09951682b826262f9861ee932a2f98c6b41ec52..4bc11c5277542b256a68234983f7e7c56f3d38c4 100644
--- a/docs/mac_build_instructions.md
+++ b/docs/mac_build_instructions.md
@@ -143,16 +143,28 @@ With hybrid builds, compilation is still handled by ninja, and can be run by the
 command line (e.g. ninja -C out/Debug chrome) or by choosing the chrome target
 in the hybrid workspace and choosing build.
 
-To use Xcode-Ninja Hybrid, set `GYP_GENERATORS=ninja,xcode-ninja`.
+To use Xcode-Ninja Hybrid, set GYP\_GENERATORS like the following:
+```

[sdefresne, 2016/03/02 14:12:18]

nit: for proper syntax highlighting, use 

```shell

[tfarina, 2016/03/02 17:17:22]

Done.

+$ export GYP_GENERATORS="ninja,xcode-ninja"

[sdefresne, 2016/03/02 14:12:18]

nit: the other pages do not prefix shell commands by $ and I think it is better
(as you can just copy-n-paste them as is)

[tfarina, 2016/03/02 17:17:22]

Done.

+```
 
 Due to the way Xcode parses ninja output paths, it's also necessary to change
 the main gyp location to anything two directories deep. Otherwise Xcode build
-output will not be clickable. Adding
-`xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp` to your
-`GYP_GENERATOR_FLAGS` will fix this.
+output will not be clickable. Exporting your GYP\_GENERATOR\_FLAGS like the
+following will fix it:
+```

[sdefresne, 2016/03/02 14:12:18]

ditto about ```shell and $

[tfarina, 2016/03/02 17:17:22]

Done.

+$ export GYP_GENERATOR_FLAGS="xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp"
+```

[sdefresne, 2016/03/02 14:12:18]

I would add the following:

To make this change permanent, you can edit `chromium.gyp_env` (or create it if
it does not exists) and define GYP\_GENERATOR\_FLAGS. In general, to use hybrid
mode your `chromium.gyp_env` could contain the following:

```json
{
  "GYP_GENERATORS"     : "ninja,xcode-ninja",
  "GYP_GENERATOR_FLAGS":
      "xcode_project_version=3.2 " +
      "xcode_ninja_main_gyp=src/build/ninja/all.ninja.gyp",
}
```

[tfarina, 2016/03/02 17:17:22]

Done.

 
-After generating the project files with gclient runhooks, open
-`src/build/ninja/all.ninja.xcworkspace`.
+After, generate the project files with:
+```

[sdefresne, 2016/03/02 14:12:18]

ditto about ```shell and $

[tfarina, 2016/03/02 17:17:22]

Done.

+$ gclient runhooks
+```
+
+And finally, open it:
+```
+$ open build/ninja/all.ninja.xcworkspace

[sdefresne, 2016/03/02 14:12:18]

ditto about ```shell and $

[tfarina, 2016/03/02 17:17:22]

Done.

+```
 
 You may run into a problem where http://YES is opened as a new tab every time
 you launch Chrome. To fix this, open the scheme editor for the Run scheme,