Update creating-extensions.md

[itsderek]

Slight change to custom animation documentation based on testing. The default animation "slide_in_left" is only available on the "join" action for character's, however it does not end in "in". I created my custom animation according to the documentation resulting in an "update" animation, however after adding "_in\" to the middle of the filename it was added to the list of options for "join".

[Jowan-Spooner]

Thanks. This is likely to change in the future, but your changes are correct, so I'll merge them!