regexp: fix copy-paste typo on Regexp.UnmarshalText doc

regexp: fix copy-paste typo on Regexp.UnmarshalText doc

I noticed that https://go.dev/cl/479401 called both methods MarshalText
in the godoc, so fix that.

While here, add more godoc links for better usability.

Change-Id: I8f10bafeca6a1ca1c1ed9be7a7dd9fdecfe991a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/484335
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>