Update README.md

Co-authored-by: Brigitta Sipőcz <b.sipocz@gmail.com>

Author: rossbar

README.md

@@ -49,7 +49,7 @@ See the linked documentation for further details.
 ```
 
 [jb-docs]: https://jupyterbook.org/stable/
-[mystmd]:
+[mystmd]: https://mystmd.org/
 
 #### Quickstart