Update stale comment

examples/option_field_ignore_empty.proto

@@ -25,8 +25,8 @@ message SubmitFeedbackRequest {
   string feedback = 1;
   // email is the email.
   string email = 2 [
-    // `ignore_empty` skips validation on a field if it's empty.
-    // The email will be not validated if it's empty.
+    // `IGNORE_IF_UNPOPULATED` skips validation on a field if it's unpopulated.
+    // The email will be not validated if it's the empty string.
     (buf.validate.field).ignore = IGNORE_IF_UNPOPULATED,
     // `string.email` validates that a string field is a valid email.
     (buf.validate.field).string.email = true