Fix tagging example

Tagging should be demonstrated using the multiple select, as that
is where it really shines. It's also how we used to demonstrate it.

This closes https://github.com/select2/select2/issues/2901.

docs/examples.html

@@ -466,9 +466,10 @@ function log (name, args) {
       </p>
 
       <p>
-        <select class="js-example-tags form-control">
+        <select class="js-example-tags form-control" multiple="multiple">
-          <option value="one">First</option>
+          <option selected="selected">orange</option>
-          <option value="other">Other</option>
+          <option>white</option>
+          <option selected="selected">purple</option>
         </select>
       </p>