Corrections for callback sections of README.md

[MasonM]

The define function no longer accepts a callback as a third parameter. This was changed in 71ca799f5fd33cf5d2525a67488e9f1018135436, but it seems this section of the README was overlooked when the docs were updated.

I also removed a little redundancy in the "Callable" section.

[GrahamCampbell]

Thanks. :)