docs($animate): fixed typo ("an animations" -> "any animations")

Closes angular#13020

Author: RaghavRamesh

src/ng/animate.js

@@ -285,7 +285,7 @@ var $AnimateProvider = ['$provide', function($provide) {
      * when an animation is detected (and animations are enabled), $animate will do the heavy lifting
      * to ensure that animation runs with the triggered DOM operation.
      *
-     * By default $animate doesn't trigger an animations. This is because the `ngAnimate` module isn't
+     * By default $animate doesn't trigger any animations. This is because the `ngAnimate` module isn't
      * included and only when it is active then the animation hooks that `$animate` triggers will be
      * functional. Once active then all structural `ng-` directives will trigger animations as they perform
      * their DOM-related operations (enter, leave and move). Other directives such as `ngClass`,