[Android] Fix uirobot --minutes help text in test_runner.py

BUG=428729

Review URL: https://codereview.chromium.org/905813002

Cr-Commit-Position: refs/heads/master@{#315120}

build/android/test_runner.py

@@ -529,7 +529,7 @@ def AddUirobotTestOptions(parser):
                      help='APK to run tests on.')
   group.add_argument(
       '--minutes', default=5, type=int,
-      help='Number of minutes to run uirobot test [default: %default].')
+      help='Number of minutes to run uirobot test [default: %(default)s].')
 
   AddCommonOptions(parser)
   AddDeviceOptions(parser)