feat: Support correct styling on pypi.org

It was styled wrong because the file extension of README.MD is set to uppercase... for whatever reason this needs to be lower case (so README.md).

Author: Dominik Falkner

{{ cookiecutter.__package_name_kebab_case }}/pyproject.toml

@@ -11,7 +11,7 @@ description = "{{ cookiecutter.package_description }}"
 authors = [
   "{{ cookiecutter.author_name }} <{{ cookiecutter.author_email }}>"
 ]
-readme = "README.MD"
+readme = "README.md"
 repository = "{{ cookiecutter.package_url }}"
 packages = [
   { include = "{{cookiecutter.__package_name_snake_case}}" }