Update finite_diff.py

Fixed typos.
Merge pull request #7794 from naslundx/patch-1

Update finite_diff.py
Add Marcus Näslund to AUTHORS/aboutus. Welcome to SymPy
Fixed minor errors in code
Merge branch 'master' of https://github.com/sympy/sympy into orienters
Merge branch 'orienters' of https://github.com/srjoglekar246/sympy into orienters