Update component-slots.md

vuejs/docs@b6070fe#diff-35274a894b839664d9b049b61f4ceb59208a220f0008127c91fd80249d07a0ec

Author: naokie

src/guide/component-slots.md

@@ -264,7 +264,7 @@ app.component('todo-list', {
 ```html
 <ul>
   <li v-for="( item, index ) in items">
-    <slot v-bind:item="item"></slot>
+    <slot :item="item"></slot>
   </li>
 </ul>
 ```