git-codereview: update help text to add -trybot
git-codereview: update help text to add -trybot

It was added to the godoc in https://golang.org/cl/24960, but that CL
missed the help text given in "git-codereview help". Briefly mention
it there too.

Change-Id: I6905b3f98f4092ed34609596bb875e88cb00d480
Reviewed-on: https://go-review.googlesource.com/41693
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/review