Update docs.yml

Ensuring this is run on the main branch

.github/workflows/docs.yml

@@ -7,7 +7,7 @@ name: Deploy docs to Pages
 on:
   # Runs on pushes targeting the default branch
   push:
-    branches: [$default-branch]
+    branches: master
 
 # Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
 permissions: