fix branch names for gh-actions

.github/workflows/mkdocs.yml

@@ -3,14 +3,14 @@ name: Publish docs via GitHub Pages
 on:
   push:
     branches:
-      - master
+      - main
 
 jobs:
   build:
     name: Deploy docs
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout master
+      - name: Checkout main
         uses: actions/checkout@v1
 
       - name: Deploy docs