Fix format indicator of README for PyPI (#376)

It was still set as rst so the PyPI page was wrongly formatted. Change
to markdown to fix the issue.

setup.cfg

@@ -3,7 +3,7 @@ name = harmonica
 fullname = Harmonica
 description = "Forward modeling, inversion, and processing gravity and magnetic data"
 long_description = file: README.md
-long_description_content_type = text/x-rst
+long_description_content_type = text/markdown
 author = The Harmonica Developers
 author_email = [email protected]
 maintainer = "Santiago Soler"