Tweak README heading

Author: justinmayer

README.md

@@ -1,5 +1,5 @@
-Use Markdown-Include extension: A Plugin for Pelican
-====================================================
+Markdown Include: A Plugin for Pelican
+======================================
 
 [![Build Status](https://img.shields.io/github/actions/workflow/status/pelican-plugins/markdown-include/main.yml?branch=main)](https://github.com/pelican-plugins/markdown-include/actions)
 [![PyPI Version](https://img.shields.io/pypi/v/pelican-markdown-include)](https://pypi.org/project/pelican-markdown-include/)