Update component-basics.md - wrong highlight about slot (#3142)

Update component-basics.md

In line 471, the line 4 is highlighted but that's wrong and the line 5 must be highlighted that is about `slot`.

src/guide/essentials/component-basics.md

@@ -468,7 +468,7 @@ Something bad happened.
 
 This can be achieved using Vue's custom `<slot>` element:
 
-```vue{4}
+```vue{5}
 <!-- AlertBox.vue -->
 <template>
   <div class="alert-box">