Fix package long description

Author: codingjoe

setup.cfg

@@ -3,7 +3,7 @@ name = django-dynamic-filenames
 author = Johannes Hoppe
 author-email = info@johanneshoppe.com
 summary = Write advanced filename patterns using the Format String Syntax.
-long-description = file: README.rst
+long_description = file: README.rst
 long_description_content_type = text/x-rst
 home-page = https://github.com/codingjoe/django-dynamic-filenames
 project_urls =