docs(API Reference): corrected two typos - two missing commas

In the ngAnimate section, there were two commas missing from two sentences. This is inconsistent with the grammar used in the rest of the API documentation and made the document (slightly) more difficult to read. The two sentences are shown below, with the new commas added:

1. "Once defined, the animation can be triggered"
                           ^
                    comma added

2. "Once registered, the animation can be triggered"
                              ^
                    comma added

Closes #10447

docs/content/api/index.ngdoc

@@ -148,15 +148,15 @@ or JavaScript callbacks.
       {@link ngAnimate CSS-based animations}
     </td>
     <td>
-      Follow ngAnimate’s CSS naming structure to reference CSS transitions / keyframe animations in AngularJS. Once defined the animation can be triggered by referencing the CSS class within the HTML template code.
+      Follow ngAnimate’s CSS naming structure to reference CSS transitions / keyframe animations in AngularJS. Once defined, the animation can be triggered by referencing the CSS class within the HTML template code.
     </td>
   </tr>
   <tr>
     <td>
       {@link ngAnimate JS-based animations}
     </td>
     <td>
-      Use {@link angular.Module#animation module.animation()} to register a JavaScript animation. Once registered the animation can be triggered by referencing the CSS class within the HTML template code.
+      Use {@link angular.Module#animation module.animation()} to register a JavaScript animation. Once registered, the animation can be triggered by referencing the CSS class within the HTML template code.
     </td>
   </tr>
 </table>