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