fix: Update sde_example.md

[DoktorMike]

I believe there's a small mistake in this example as the equation uses σ but in the code we use θ (in two places instead of one).

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

I fixed the consistency with the equations but the functions don't actually make much sense as I see it as f and g should really be using p instead of some global variables μ, κ, θ, σ if I'm not mistaken.

[ChrisRackauckas]

Yeah, the:

Of course, to fully define this problem, we need to define our constants.

is doing a lot of heavy lifting. We should probably spend a moment sometime in the near future to really get that in good shape.