fix: an error expression in demo template

mvvm.html

@@ -14,7 +14,7 @@
         <span v-text="child.someStr"></span>
     </p> -->
     <p>{{getHelloWord}}</p>
-    <p v-html="child.htmlStr"></p>
+    <p v-html="htmlStr"></p>
     <button v-on:click="clickBtn">change model</button>
 </div>