doc: use <code> for a few file paths that didn't

Spotted while seeing CL 187818 in master.

Change-Id: Ic3f42558f6833fc35580b2b2a15f2c5aba5713c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/187822


Reviewed-by: Ian Lance Taylor <iant@golang.org>