fix(tutorial): race condition on step-10 (vuejs#2567)

Author: ubugeeei

src/tutorial/src/step-10/App/template.html

@@ -1,4 +1,4 @@
 <p>Todo id: {{ todoId }}</p>
-<button @click="todoId++">Fetch next todo</button>
+<button @click="todoId++" :disabled="!todoData">Fetch next todo</button>
 <p v-if="!todoData">Loading...</p>
 <pre v-else>{{ todoData }}</pre>