.github: update PULL_REQUEST_TEMPLATE to match the latest instruction

Update the wiki link too.

Change-Id: Ic3a780fa7fbdc7a7086e1a7f55932ab218d3fad9
Reviewed-on: https://go-review.googlesource.com/c/go/+/782000
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>