fix(data/fintype/basic): fix `fintype_of_option_equiv` (#13466)
fix(data/fintype/basic): fix `fintype_of_option_equiv` (#13466)

A type is a `fintype` if its successor (using `option`) is a `fintype`

This fixes an error introduced in #13086.





leanprover-community/mathlib,leanprover-community/mathlib