fix a README link

docs/updating_clang_format_binaries.md

 # Updating Clang format binaries
 
 Instructions on how to update the [clang-format binaries](clang_format.md) that
 come with a checkout of Chromium.
 
 ## Prerequisites
 
 You'll need a Windows machine, a Linux machine, and a Mac; all capable of
 building clang-format. You'll also need permissions to upload to the appropriate
 google storage bucket. Chromium infrastructure team members have this, and
@@ skipping 73 matching lines @@
     libatomic.so.1 which doesn't exist on Precise.
 *   Mac: Remember to set `MACOSX_DEPLOYMENT_TARGET` when building! If you get
     configure warnings, you may need to install XCode 5 and avoid a goma
     environment.
 
 ## Upload each binary to google storage
 
 Copy the binaries into your chromium checkout (under
 `src/buildtools/(win|linux64|mac)/clang-format(.exe?)`). For each binary, you'll
 need to run upload_to_google_storage.py according to the instructions in
-[README.txt](/buildtools/clang_format/README.txt). This will upload the binary
-into a publicly accessible google storage bucket, and update `.sha1` file in
-your Chrome checkout. You'll check in the `.sha1` file (but NOT the clang-format
-binary) into source control. In order to be able to upload, you'll need write
-permission to the bucket -- see the prerequisites.
+[README.txt](https://chromium.googlesource.com/chromium/buildtools/+/master/clang_format/README.txt).
+This will upload the binary into a publicly accessible google storage bucket,
+and update `.sha1` file in your Chrome checkout. You'll check in the `.sha1`
+file (but NOT the clang-format binary) into source control. In order to be able
+to upload, you'll need write permission to the bucket -- see the prerequisites.
 
 ## Copy the helper scripts and update README.chromium
 
 There are some auxiliary scripts that ought to be kept updated in lockstep with
 the clang-format binary. These get copied into
 third_party/clang_format/scripts in your Chromium checkout.
 
 The `README.chromium` file ought to be updated with version and date info.
 
 ## Upload a CL according to the following template
 
     Update clang-format binaries and scripts for all platforms.
 
     I followed these instructions:
     https://chromium.googlesource.com/chromium/src/+/master/docs/updating_clang_format_binaries.md
 
     The binaries were built at clang revision ####### on ####DATETIME####.
 
     BUG=
 
 The change should **always** include new `.sha1` files for each platform (we
 want to keep these in lockstep), should **never** include `clang-format`
 binaries directly. The change should **always** update `README.chromium`
 
 clang-format binaries should weigh in at 1.5MB or less. Watch out for size
 regressions.