cmd/gorename: make Usage documentation sensible in godoc

Fixes golang/go#9469

Change-Id: I74e19e3d095e4c2ab62f755a408f58fd5e7ac2f2
Reviewed-on: https://go-review.googlesource.com/2176
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
3 files changed