[COIL] Change chromium code search type links to use main rather than…

… master for buildtools

    See this doc: go/chromium-coil-change for more info
    BUG=1210385

Change-Id: I9a865daa91b424d4cb2788d6cefa1a7f4561b3a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2902076
Auto-Submit: John Palmer <jopalmer@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#884602}

buildtools/clang_format/README.txt

@@ -2,7 +2,7 @@ This folder contains clang-format scripts. The binaries will be automatically
 downloaded from Google Storage by gclient runhooks for the current platform.
 
 For a walkthrough on how to maintain these binaries:
-  https://chromium.googlesource.com/chromium/src/+/master/docs/updating_clang_format_binaries.md
+  https://chromium.googlesource.com/chromium/src/+/main/docs/updating_clang_format_binaries.md
 
 To upload a file:
   python ~/depot_tools/upload_to_google_storage.py -b chromium-clang-format <FILENAME>