Update README.md

Author: rosscdh

README.md

@@ -27,7 +27,7 @@ Enable the plugin in your `mkdocs.yml`:
 ```yaml
 plugins:
     - search
-    - markdownextra
+    - markdownextradata
 ```
 
 > **Note:** If you have no `plugins` entry in your config file yet, you'll likely also want to add the `search` plugin. MkDocs enables it by default if there is no `plugins` entry set, but now you have to enable it explicitly.