docs: make pinPadding value change reflect in the template

vuejs/docs@c4b734d#diff-91618a748fbcef0b7ce1f26f300d5aabe925114da5a69022eef7d42a7ed4d3bf

Author: naokie

src/guide/custom-directive.md

@@ -130,7 +130,7 @@ app.mount('#dynamic-arguments-example')
 <div id="dynamicexample">
   <h2>Scroll down the page</h2>
   <input type="range" min="0" max="500" v-model="pinPadding">
-  <p v-pin:[direction]="pinPadding">Stick me 200px from the {{ direction }} of the page</p>
+  <p v-pin:[direction]="pinPadding">Stick me {{ pinPadding + 'px' }} from the {{ direction }} of the page</p>
 </div>
 ```