gopls: update go.mod in preparation for 0.2.2 release

Sync to x/tools CL 210204.

Change-Id: I02b9d4401f1af8d93f96449b069ee809245c5f01
Reviewed-on: https://go-review.googlesource.com/c/tools/+/210205
Reviewed-by: Heschi Kreinick <heschi@google.com>

gopls/go.mod

@@ -5,7 +5,7 @@ go 1.11
 require (
 	github.com/sergi/go-diff v1.0.0
 	github.com/stretchr/testify v1.4.0 // indirect
-	golang.org/x/tools v0.0.0-20191112202413-08e24063958e
+	golang.org/x/tools v0.0.0-20191206190119-8f1694653207
 	honnef.co/go/tools v0.0.1-2019.2.3
 )