encoding/json: spell "marshaling" and "unmarshaling" consistently

Fixes #12431

Change-Id: I67c42bf2cd9285f471387248fd9c22a16b158349
Reviewed-on: https://go-review.googlesource.com/14150
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/src/encoding/json/decode_test.go b/src/encoding/json/decode_test.go
index 8c3b92b..e9e00e5 100644
--- a/src/encoding/json/decode_test.go
+++ b/src/encoding/json/decode_test.go
@@ -1206,12 +1206,12 @@
 
 	data, err := Marshal(m1)
 	if err != nil {
-		t.Errorf("Unexpected error marshalling: %v", err)
+		t.Errorf("Unexpected error marshaling: %v", err)
 	}
 
 	err = Unmarshal(data, &m2)
 	if err != nil {
-		t.Errorf("Unexpected error unmarshalling: %v", err)
+		t.Errorf("Unexpected error unmarshaling: %v", err)
 	}
 
 	if !reflect.DeepEqual(m1, m2) {