docs(ngAnimate): fix toggle button in example

The toggle button code on line 153 only sets bool to true, rather than toggling it.
The proposed change fixes this.

Closes #14387

src/ngAnimate/module.js

@@ -130,7 +130,7 @@
  * <div ng-show="bool" class="fade">
  *   Show and hide me
  * </div>
- * <button ng-click="bool=true">Toggle</button>
+ * <button ng-click="bool=!bool">Toggle</button>
  *
  * <style>
  * .fade.ng-hide {