descriptor: fix example name (#559)

Example_Options is an example on the Options type or function.
Example_options is an example on the package with a suffix of "options".

descriptor/descriptor_test.go

@@ -20,7 +20,7 @@ func TestMessage(t *testing.T) {
 	}
 }
 
-func Example_Options() {
+func Example_options() {
 	var msg *tpb.MyMessageSet
 	_, md := descriptor.ForMessage(msg)
 	if md.GetOptions().GetMessageSetWireFormat() {