Set the right long description type

Author: tm-kn

setup.cfg

@@ -3,7 +3,7 @@ name = django-basic-auth-ip-whitelist
 version = attr: baipw.__version__
 description = Hide your Django site behind basic authentication mechanism with IP whitelisting support.
 long-description = file: README.rst
-long-description-content-type = text/rst
+long-description-content-type = text/x-rst
 author = Tomasz Knapik
 author_email = hi@tmkn.org
 url = https://github.com/tm-kn/django-basic-auth-ip-whitelist