Bugfix. Add missing variable for string interpolation

src/headless/converse-muc.js

@@ -2053,7 +2053,7 @@ converse.plugins.add('converse-muc', {
                 if (code === '301') {
                     return actor ? __("%1$s has been banned by %2$s", nick, actor) : __("%1$s has been banned", nick);
                 } else if (code === '303') {
-                    return ___("%1$s's nickname has changed");
+                    return ___("%1$s's nickname has changed", nick);
                 } else  if (code === '307') {
                     return actor ? __("%1$s has been kicked out by %2$s", nick, actor) : __("%1$s has been kicked out", nick);
                 } else if (code === '321') {