encoding/protobuf: fix required option parsing

Fixes #351

Change-Id: Iaa14fee036764c7a51116483ea51897c0ef87efd
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5700
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/parse.go b/encoding/protobuf/parse.go
index 5902bfb..9cb342b 100644
--- a/encoding/protobuf/parse.go
+++ b/encoding/protobuf/parse.go
@@ -756,7 +756,7 @@
 	// - interpret CUE options.
 	for _, o := range options {
 		switch o.Name {
-		case "(cue_opt).required":
+		case "(cue.opt).required":
 			p.required = true
 			// TODO: Dropping comments. Maybe add a dummy tag?