Correct example docs for Anima#add and #remove

Looks like these commits first moved methods around, then got the doc-example comments a little out of whack.

https://github.com/mbj/anima/commit/d73dba18b51742be0c07046dd6370b37c47538e7
https://github.com/mbj/anima/commit/2bb9776ccec77f823be592ac98b31a8d4ad576b0