Standardize on space between "RFC" and number. Additionally change
the couple "a RFC" instances to "an RFC."

Fixes #15258

Change-Id: I2b17ecd06be07dfbb4207c690f52a59ea9b04808
Reviewed-on: https://go-review.googlesource.com/21902Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>