docs: update for v3

Author: peaceiris

README.md

@@ -811,8 +811,8 @@ on:
       - master
 
 jobs:
-  build-deploy:
-    runs-on: ubuntu-latest
+  deploy:
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
 
@@ -830,11 +830,10 @@ jobs:
         # provide --output=<output-file> option for `elm make` and remove this step
 
       - name: Deploy
-        uses: peaceiris/actions-gh-pages@v2
-        env:
-          ACTIONS_DEPLOY_KEY: ${{ secrets.ACTIONS_DEPLOY_KEY }}
-          PUBLISH_BRANCH: gh-pages
-          PUBLISH_DIR: ./public
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}
+          publish_dir: ./public
 ```
 
 ## License