Add dependency to HTML (GoogleCloudPlatform#6552)

## Description

Fixes #<ISSUE-NUMBER>

Note: It's a good idea to open an issue first for discussion.

## Checklist
- [ ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: averikitsch

appengine/standard_python3/building-an-app/building-an-app-3/templates/index.html

@@ -35,6 +35,7 @@
       alert(msg);
     }
   </script>
+  <script src="https://www.gstatic.com/firebasejs/8.9.1/firebase-auth.js"></script>
   <!-- [START gae_python38_auth_include_firebaseui] -->
   <!-- [START gae_python3_auth_include_firebaseui] -->
   <script src="https://www.gstatic.com/firebasejs/ui/4.4.0/firebase-ui-auth.js"></script>