[release-branch.go1.12] doc: fix typo in Go 1.12.14 document

Change-Id: I3641a086f167a1337aaaacd2d758b6a42b84a7fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/209845


Run-TryBot: Carlos Amedee <carlos@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 98e7270a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/209846