changed theme and plugin to headings (#786)

Co-authored-by: Tatiana <[email protected]>

Author: tjperry07

demo/docs/intro.mdx

@@ -96,13 +96,13 @@ yarn start
 Both the plugin and theme are currently designed to pair with a specific Docusaurus release. The Docusaurus badge in the `README.md` and at the top of this page will always reflect the current compatible versions.
 :::
 
-Plugin:
+### Plugin
 
 ```bash
 yarn add docusaurus-plugin-openapi-docs
 ```
 
-Theme:
+### Theme
 
 ```bash
 yarn add docusaurus-theme-openapi-docs