Update go.mod

Signed-off-by: John Hwang <john.f.hwang@gmail.com>

vendor/google.golang.org/protobuf/encoding/prototext/doc.go

@@ -0,0 +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 prototext marshals and unmarshals protocol buffer messages as the
+// textproto format.
+package prototext