gopls: update go.mod for gopls/v0.4.0-pre2

Change-Id: Ica799fcf117bf607c0c59f41b08a78552339dc53
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227356
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/gopls/go.mod b/gopls/go.mod
index 1f9c75a..9868ad5 100644
--- a/gopls/go.mod
+++ b/gopls/go.mod
@@ -4,7 +4,7 @@
 
 require (
 	github.com/sergi/go-diff v1.1.0
-	golang.org/x/tools v0.0.0-20200403192435-a2d82aa460de
+	golang.org/x/tools v0.0.0-20200406211959-b2e691980a9d
 	honnef.co/go/tools v0.0.1-2020.1.3
 	mvdan.cc/xurls/v2 v2.1.0
 )