Fix typo in build_directory.py

Fix a typo that causes sizes.py to fail on Mac.

Bug: 1097180
Change-Id: I5d375aa25d6fe9925b889bbd3ea7b061bd0f7473
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2330823
Auto-Submit: John Chen <johnchen@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793647}

infra/scripts/build_directory.py

@@ -52,7 +52,7 @@ def GetBuildOutputDirectory(src_dir=None, cros_board=None):
   assert not cros_board, "'cros_board' not supported on this platform"
 
   if sys.platform == 'cygwin' or sys.platform.startswith('win') or (
-      sys.platorm == 'darwin'):
+      sys.platform == 'darwin'):
     return os.path.join(src_dir, 'out')
 
   raise NotImplementedError('Unexpected platform %s' % sys.platform)