Document the parameter {heading} avilable to --output-format-heading.

R=jsbell@chromium.org

Change-Id: Ia0db8fe7494306147e0e80f49e229f50b8603f09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2214243
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Vincent Scheib <scheib@chromium.org>

my_activity.py

@@ -795,7 +795,8 @@ def main():
   output_format_group.add_option(
       '--output-format-heading', metavar='<format>',
       default=u'{heading}:',
-      help='Specifies the format to use when printing headings.')
+      help='Specifies the format to use when printing headings. '
+           'Supports the variable {heading}.')
   output_format_group.add_option(
       '--output-format-no-url', default='{title}',
       help='Specifies the format to use when printing activity without url.')