Update to point to main rather than master

Author: mmcky

lectures/_config.yml

@@ -49,6 +49,7 @@ sphinx:
       header_organisation_url: https://quantecon.org
       header_organisation: QuantEcon
       repository_url: https://github.com/QuantEcon/lecture-julia.myst
+      repository_branch: main
       nb_repository_url: https://github.com/QuantEcon/lecture-julia.notebooks
       twitter: quantecon
       twitter_logo_url: https://assets.quantecon.org/img/qe-twitter-logo.png