Per note at top of doc, we don't use fixed-width spaces
in fixed-width phrases like "go doc".

Also ASN.1 NULL is not code so it's not <code> at all.

Change-Id: I791e4e6030b8b8d42f4621d2f4bf32fef93cf343
Reviewed-on: https://go-review.googlesource.com/47693
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>