Correct documentation for the --no-referenced-issues argument

When specified it will skip any issues that were not modified in the specified
time period even if they are owned by the user.

R=tandrii@chromium.org

Change-Id: I230c8fa6cbc517c32edb8618a831a62ec6e45405
Reviewed-on: https://chromium-review.googlesource.com/978170
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

my_activity.py

@@ -849,7 +849,7 @@ def main():
       action='store_true',
       help='Do not fetch issues referenced by owned changes. Useful in '
            'combination with --changes-by-issue when you only want to list '
-           'issues that are your own in the output.')
+           'issues that have also been modified in the same time period.')
   parser.add_option(
       '--skip-own-issues-without-changes',
       action='store_true',