Fix Decorder doc typo (#494)

yaml.go

@@ -89,7 +89,7 @@ func UnmarshalStrict(in []byte, out interface{}) (err error) {
 	return unmarshal(in, out, true)
 }
 
-// A Decorder reads and decodes YAML values from an input stream.
+// A Decoder reads and decodes YAML values from an input stream.
 type Decoder struct {
 	strict bool
 	parser *parser