docs(ngAnimate): fix typo ("an the" --> "an")

Fixes #15194

src/ngAnimate/module.js

@@ -37,7 +37,7 @@
  * ## CSS-based Animations
  *
  * CSS-based animations with ngAnimate are unique since they require no JavaScript code at all. By using a CSS class that we reference between our HTML
- * and CSS code we can create an animation that will be picked up by Angular when an the underlying directive performs an operation.
+ * and CSS code we can create an animation that will be picked up by Angular when an underlying directive performs an operation.
  *
  * The example below shows how an `enter` animation can be made possible on an element using `ng-if`:
  *