PRESUBMIT: Fix typo.

In https://webrtc-codereview.appspot.com/50069004/
the regex for Mac trybots became invalid. This fixes it.

TBR=phoglund@webrtc.org
TESTED=Ran 'git cl try --bot=mac_x64_gn'

Review URL: https://webrtc-codereview.appspot.com/53529004

Cr-Commit-Position: refs/heads/master@{#9278}
1 file changed