oauth2: update "review" to "codereview" in CONTRIBUTING

Ticks have also been added around commands for style.

Change-Id: Ie2f1934503f73a106cc60d8f9e122dcee69bd000
Reviewed-on: https://go-review.googlesource.com/2130
Reviewed-by: Andrew Gerrand <adg@golang.org>
oauth2: remove duplicate package doc and import comments

The comments already live in oauth2.go.

Change-Id: I103113fbc8138b242cb6f79052fdc6b29bf6f031
Reviewed-on: https://go-review.googlesource.com/2131
Reviewed-by: Andrew Gerrand <adg@golang.org>