gopls: update go.mod for gopls/v0.4.0-pre1

Change-Id: Ic0b610b3293e118b2fa5180234bdc1142b7e052c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227140
Reviewed-by: Rohan Challa <rohan@golang.org>
diff --git a/gopls/go.mod b/gopls/go.mod
index 12d68af..1f9c75a 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 v0.0.0-20191130070609-6e064ea0cf2d
+	golang.org/x/tools v0.0.0-20200403192435-a2d82aa460de
 	honnef.co/go/tools v0.0.1-2020.1.3
 	mvdan.cc/xurls/v2 v2.1.0
 )