Update main.yml

Changed `master` branch to `main` branch.

.github/workflows/main.yml

@@ -2,14 +2,14 @@ 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