cmd/digraph: fix package doc

The copyright statement is leaked to the package doc.

Change-Id: Iace97037f3bf97ab7a4a1ad64ddcc86f56b4431f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/752200
Reviewed-by: Ethan Lee <ethanalee@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: hyangah

cmd/digraph/digraph.go

@@ -1,6 +1,7 @@
 // Copyright 2019 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package main // import "golang.org/x/tools/cmd/digraph"
 
 // TODO(adonovan):