RTL: fix domain append at signup form (#9007)

app/javascript/styles/mastodon/rtl.scss

@@ -219,7 +219,7 @@ body.rtl {
     padding-right: 0;
   }
 
-  .simple_form .input-with-append .append {
+  .simple_form .label_input__append {
     right: auto;
     left: 0;