gopls: update go.mod for gopls/v0.5.1-pre3

Change-Id: I7a09fde6c50aa6d648c8288ba8b4eabb7ee32e15
Reviewed-on: https://go-review.googlesource.com/c/tools/+/258523
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>

gopls/go.mod

@@ -4,7 +4,7 @@ go 1.12
 
 require (
 	github.com/sergi/go-diff v1.1.0
-	golang.org/x/tools v0.0.0-20200929211117-1e1ba560f0b0
+	golang.org/x/tools v0.0.0-20200930165741-f1523d29dbb9
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
 	honnef.co/go/tools v0.0.1-2020.1.5
 	mvdan.cc/gofumpt v0.0.0-20200802201014-ab5a8192947d