go/doc: allow interior dot in heading, as in "go.mod"

Only the expected headings are affected.
Diffing the output of "go run headscan.go" before and after:

$ diff head1 head2
26a27,28
> 	Edit go.mod from tools or scripts
> 	Make go.mod semantically consistent
168c170
< 141 headings found
---
> 143 headings found
$

Fixes #26938.

Change-Id: I204fd982a60773aa26880cd19eed890c373b8ab6
Reviewed-on: https://go-review.googlesource.com/129677


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>