fix: i18ze a string (#3210)

Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/3210
Reviewed-by: konrad <k@knt.li>
Co-authored-by: WofWca <wofwca@protonmail.com>
Co-committed-by: WofWca <wofwca@protonmail.com>

src/views/tasks/TaskDetailView.vue

@@ -731,7 +731,7 @@ async function saveTask(args?: {
 	let actions = []
 	if (undoCallback !== null) {
 		actions = [{
-			title: 'Undo',
+			title: t('task.undo'),
 			callback: undoCallback,
 		}]
 	}