gopls: update go.mod
gopls: update go.mod

Change-Id: Ie42b99e23d94d8a5f745cbfb81d943f7a4d73867
Reviewed-on: https://go-review.googlesource.com/c/tools/+/195657
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools