_content/doc: fix go mod edit link in mod.md
_content/doc: fix go mod edit link in mod.md

Change-Id: I979dcf6b9e097abc70affdf222fd5b7e296a3fd2
Reviewed-on: https://go-review.googlesource.com/c/website/+/291869
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website