docs: Modify Google and GitHub instructions for auth setup.

In auth setup, if you're at the error page you may be inclined to
refresh the page once setting up auth after restarting the server,
but the error page is a static page, so add instructions to go
back to the login pages again and try the auth flow from there.

templates/zerver/github-error.md

@@ -8,3 +8,5 @@ in the OAuth application in GitHub. You can create OAuth apps from
 * You have set `SOCIAL_AUTH_GITHUB_KEY` in `{{ settings_path }}` and
 `social_auth_github_secret` in `{{ secrets_path }}` with the values
 from your OAuth application.
+
+* Navigate back to the login page and attempt the GitHub auth flow again.

templates/zerver/google-error.md

@@ -9,3 +9,5 @@ server's Google auth URL: `{{ server_uri }}/accounts/login/google/done/`.
 
 * You have set `GOOGLE_OAUTH2_CLIENT_ID` in `{{ settings_path }}` and
 `google_oauth2_client_secret` in `{{ secrets_path }}`.
+
+* Navigate back to the login page and attempt the Google auth flow again.