wiki: fix link for sending a change when updating existing content

Change-Id: I6ab5fbe2649d69893314559d2e9661eaf6d3f049
GitHub-Last-Rev: c468dd2786319d639e145e7c560345a77be1e851
GitHub-Pull-Request: golang/wiki#31
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/621918
Commit-Queue: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>