Main branch fix

Author: jorgensd

_config.yml

@@ -14,7 +14,8 @@ execute:
 # Information about where the book exists on the web
 repository:
   url: https://github.com/scientificcomputing/example-paper-fenics # Online location of your book
- 
+  branch: main
+
 launch_buttons:
   notebook_interface: "jupyterlab" # The interface interactive links will activate ["classic", "jupyterlab"]
   binderhub_url: "https://mybinder.org"