presubmit: Improve CheckInfraFreeze error message

I didn't understand from the error message how to correctly set the git-footer in my commit message, so that the override takes effect.

Maybe this version is easier for new devs?

Change-Id: I745d80f0e8068500137d9382f423d0c65a922f5b
Ignore-Freeze: Low risk
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5930648
Auto-Submit: Philipp Wollermann <philwo@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

presubmit_canned_checks.py

@@ -970,11 +970,12 @@ def CheckInfraFreeze(input_api,
     else:
         report_type = output_api.PresubmitError
     return [
-        report_type('There is a prod infra freeze in effect from {} until {}:\n'
+        report_type(
+            'There is a prod infra freeze in effect from {} until {}:\n'
             '\t{}\n\n'
             'The following files cannot be modified:\n  {}.\n\n'
-                    'Set git-footer "{}" to override.'.format(
+            'Add "{}: <reason>" to the end of your commit message to override.'.
-                        _FREEZE_START, _FREEZE_END, _FREEZE_DETAILS,
+            format(_FREEZE_START, _FREEZE_END, _FREEZE_DETAILS,
                    '\n  '.join(files), _IGNORE_FREEZE_FOOTER))
     ]