internal/deepprint: update copyright header to appease license checker script
Plus mention that it's not an exact copy.
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// and
+
// Copyright 2018 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.
-// This is a copy of Go's src/internal/fmtsort/sort.go
+// This is a slightly modified fork of Go's src/internal/fmtsort/sort.go
package deepprint
