[infra] Fix clang-format invocation from git cl format

tools/buildtools/README.md

Index: tools/buildtools/README.md
diff --git a/tools/buildtools/README.md b/tools/buildtools/README.md
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+To build Dart for Mac and Linux, we pull Fuchsia's buildtools, which is also
+used by Flutter. Fuchsia's buildtools includes gn, ninja, and the clang
+toolchain. Fuchsia buildtools vends clang-format as part of the clang toolchain.
+Since Fuchsia's buildtools doesn't vend a clang toolchain for Windows, we can't
+get a Windows clang-format binary from it. Therefore, from Chromium's buildtools
+here:
+
+https://chromium.googlesource.com/chromium/buildtools
+
+we copy the hash file for its Windows clang-format binary, and pull it down
+from google storage in the update.py script in this directory.
+
+To update to a newer Windows clang-format binary, simply overwrite the hash in
+clang-format.exe.sha1 with a newer one, and then do a 'gclient sync'.