fix: add type annotation to step parameters in AutoFiniteDiff

[gdalle]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

#102 added the relstep and absstep parameters without type annotations, which is equivalent to ::Any. This PR fixes it.

[gdalle]

One question for @ChrisRackauckas / @Vaibhavdixit02 / @avik-pal: will these additional type parameters cause problems if someone used to write AutoFiniteDiff{T1,T2,T3} and expect a concrete type?

[gdalle]

Since this is a very small change fixing a rather big mistake in a past PR I reviewed, I hope it's okay that I merge it myself. Do let me know if it was not.