docs: replace master with main to reflect GH default (#6377)

Author: anbnyc

docs/guide/deployment.md

@@ -75,10 +75,10 @@ If you are using the PWA plugin, your app must be served over HTTPS so that [Ser
     git commit -m 'deploy'
 
     # if you are deploying to https://<USERNAME>.github.io
-    # git push -f [email protected]:<USERNAME>/<USERNAME>.github.io.git master
+    # git push -f [email protected]:<USERNAME>/<USERNAME>.github.io.git main
 
     # if you are deploying to https://<USERNAME>.github.io/<REPO>
-    # git push -f [email protected]:<USERNAME>/<REPO>.git master:gh-pages
+    # git push -f [email protected]:<USERNAME>/<REPO>.git main:gh-pages
 
     cd -
     ```
@@ -112,7 +112,7 @@ If you are using the PWA plugin, your app must be served over HTTPS so that [Ser
      github_token: $GITHUB_TOKEN
      local_dir: dist
      on:
-       branch: master
+       branch: main
     ```
 
 6. Push the `.travis.yml` file to your repository to trigger the first build.