gopls: use correct doc link

Change-Id: Iad836fa1d462a007fd5a8c096a3b7040840ee2f1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/703695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: adonovan

gopls/README.md

@@ -9,5 +9,5 @@ It provides a wide variety of [IDE features](doc/features/)
 to any [LSP](https://microsoft.github.io/language-server-protocol/)-compatible
 editor.
 
-- Documentation for [users](doc/index.md). <!-- TODO: link to go.dev/gopls in due course -->
-- Documentation for [contributors](doc/contributing.md).
+- Documentation for [users](https://go.dev/gopls)
+- Documentation for [contributors](doc/contributing.md)