Corrected content type of description

Author: gnongsie

setup.py

@@ -23,6 +23,7 @@
 
     description='Authorize.Net Python SDK',
     long_description=long_description,
+    long_description_content_type='text/markdown',
 
     # The project's main homepage.
     url='https://github.com/AuthorizeNet/sdk-python',