Updating README: rename menuBtn to btn to match options object

[bclouser]

This was drove me crazy a little bit, and took me longer than id like to admit to root-cause 😏

Another heads-up, the documentation here is also out of sync. http://franverona.com/jemoji/#documentation

Though, perhaps you wanted options.btn to become options.menuBtn???

Great addon, btw. Easy to use, right out of the box!