Update runtime.txt to Python 3.9 and fix link to myst-nb docs (#117)

* Update runtime.txt to Python 3.9

* update link to myst-nb docs for text based interface

Author: dschult

runtime.txt

@@ -1 +1 @@
-python-3.8
+python-3.9

site/index.md

@@ -26,7 +26,7 @@ You can follow along yourself:
  2. *locally*, by cloning the repository (see the octocat icon above) and
     running `jupyter notebook`.
 
-[myst-nb]: https://myst-nb.readthedocs.io/en/latest/use/markdown.html
+[myst-nb]: https://myst-nb.readthedocs.io/en/latest/authoring/text-notebooks.html
 
 ## Contents