Fix platform.system check in clang-format presubmit

I did this incorrectly in
https://dawn-review.googlesource.com/c/dawn/+/24640
because I forgot to actually look up what the platform.system() return
values were.

Bug: none
Change-Id: Ib81060d14339cf8cf52764ea779e4448ac92a148
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/24686
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 55958d3..4db937e 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -19,7 +19,7 @@
 def _DoClangFormat(input_api, output_api):
     if platform.system() == 'Linux' and platform.architecture()[0] == '64bit':
         clang_format_path = 'buildtools/linux64/clang-format'
-    elif platform.system() == 'Mac':
+    elif platform.system() == 'Darwin':
         clang_format_path = 'buildtools/mac/clang-format'
     elif platform.system() == 'Windows':
         clang_format_path = 'buildtools/win/clang-format.exe'