l10n: Changed spelling of "user name" to "username"

Using "username" like on > 200 strings over the whole Nextcloud project.

Signed-off-by: rakekniven mark.ziegler@rakekniven.de

src/components/MessagesList/MessagesGroup/MessagesGroup.vue

@@ -118,7 +118,7 @@ export default {
 			}
 
 			if (displayName === '') {
-				return t('spreed', '[Unknown user name]')
+				return t('spreed', '[Unknown username]')
 			}
 
 			return displayName