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}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index c6cb816..619a312 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -377,7 +377,7 @@
   if all(re.search(r'[\\/]BUILD.gn$', f) for f in files):
     return GetDefaultTryConfigs(android_gn_bots + linux_gn_bots + mac_gn_bots +
                                 win_gn_bots)
-  if all(re.search('[/_])mac[/_.]', f) for f in files):
+  if all(re.search('[/_]mac[/_.]', f) for f in files):
     return GetDefaultTryConfigs(mac_bots)
   if all(re.search('(^|[/_])win[/_.]', f) for f in files):
     return GetDefaultTryConfigs(win_bots)