Use clearer table header copy for /gerrit/authors

The "changes pushed" copy is slightly deceptive, in that it really means
"changes pushed that have had activity within the specified interval",
but that would've been too much of a mouthful.

Change-Id: Ia7ff873767a73d77b0a18508b964c893f6d93392
Reviewed-on: https://gerrit.causes.com/26976
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>