Make it clear that run is the default Telemetry command.
It was not previously clear from the help text that run is the default
command. This makes two changes to make it more clear: it surrounds
'command' with [] instead of <> to suggest that it's optional like the options
are, and changes the description of run to suggest that it's the default.
BUG=359791
Review URL: https://codereview.chromium.org/
418643005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285205 0039d316-1c4b-4281-b951-d872f2087c98