encoding/textpb: fix marshaling repeated group field name

In https://golang.org/cl/157821, I attempted to fix handling of group
field names but forgot about repeated group fields as I forgot to
properly update the marshaling tests as well for it.

Unmarshal logic for both repeated and non-repeated was already fixed in
that CL.

Change-Id: Icb4a00d8b169709ca12dfee272b2bd73e7585e6e
Reviewed-on: https://go-review.googlesource.com/c/157857
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/encoding/textpb/encode_test.go b/encoding/textpb/encode_test.go
index 15a5492..fe4271d 100644
--- a/encoding/textpb/encode_test.go
+++ b/encoding/textpb/encode_test.go
@@ -537,11 +537,11 @@
 				{},
 			},
 		},
-		want: `rptgroup: {
+		want: `RptGroup: {
   rpt_bool: true
   rpt_bool: false
 }
-rptgroup: {}
+RptGroup: {}
 `,
 	}, {
 		desc:  "map fields empty",