Change root from 'intro' to 'README' in _toc.yml

Author: mgough-970

_toc.yml

@@ -1,7 +1,7 @@
 # Table of contents
 # Learn more at https://jupyterbook.org/customize/toc.html
 format: jb-book
-root: intro
+root: README
 parts:
   - caption: Notebooks
     chapters: