my_activity.py prints out a help message when required, which has some
example ways to use my_activity.py. However the dates used are ambiguous
in terms of the date format used/required. This CL changes the example
to make it clearer that it is mm/dd/yy.

Change-Id: Id054739201e5c83528902b1f38d1c27c7bbc788b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1682498
Auto-Submit: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>