cmd/go: fix godoc formatting for text from 'go help install'

Fixes #44846

Change-Id: I5a12c6437a91ce59307483ffcc70e084edc32197
Reviewed-on: https://go-review.googlesource.com/c/go/+/301329


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>