docs: Fix reStructuredText error in README

Author: riddell-stan

README.rst

@@ -39,7 +39,7 @@ Similar projects
 - PyMC: http://pymc-devs.github.io/pymc/
 
 PyStan3 / Stan3
-------------
+---------------
 The development of PyStan3 with updated API can be found under `stan-dev/pystan-next <https://github.com/stan-dev/pystan-next>`_
 
 Detailed Installation Instructions

setup.py

@@ -211,6 +211,7 @@ def setup_package():
                     license=LICENSE,
                     url=URL,
                     long_description=LONG_DESCRIPTION,
+                    long_description_content_type='text/x-rst',
                     classifiers=CLASSIFIERS,
                     **extra_setuptools_args)
     if len(sys.argv) >= 2 and ('--help' in sys.argv[1:] or sys.argv[1]