encoding/prototext: document unstable marshal output

Fixes golang/protobuf#920.

Change-Id: I04c12de9a662eb67994fc7eeceee1af4a9efee55
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/188937
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/protojson/encode.go b/encoding/protojson/encode.go
index 5f383fe..fde1829 100644
--- a/encoding/protojson/encode.go
+++ b/encoding/protojson/encode.go
@@ -20,7 +20,7 @@
 )
 
 // Marshal writes the given proto.Message in JSON format using default options.
-// Do not depend on the output of being stable. It may change over time across
+// Do not depend on the output being stable. It may change over time across
 // different versions of the program.
 func Marshal(m proto.Message) ([]byte, error) {
 	return MarshalOptions{}.Marshal(m)