Enha: Bump flutter-gh-pages to v3 (#397)

Author: marandaneto

.github/workflows/web-example-ghpages.yml

@@ -11,6 +11,6 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: subosito/flutter-action@v1
-      - uses: erickzanardo/flutter-gh-pages@v2
+      - uses: erickzanardo/flutter-gh-pages@v3
         with:
           workingDir: flutter/example