Make it clear in the help message that my_activity.py dates are mm/dd/yy

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>

my_activity.py

@@ -9,8 +9,8 @@ Example:
   - my_activity.py  for stats for the current week (last week on mondays).
   - my_activity.py -Q  for stats for last quarter.
   - my_activity.py -Y  for stats for this year.
-  - my_activity.py -b 4/5/12  for stats since 4/5/12.
-  - my_activity.py -b 4/5/12 -e 6/7/12 for stats between 4/5/12 and 6/7/12.
+  - my_activity.py -b 4/24/19  for stats since April 24th 2019.
+  - my_activity.py -b 4/24/19 -e 6/16/19 stats between April 24th and June 16th.
   - my_activity.py -jd to output stats for the week to json with deltas data.
 """