Missing bundle install in testing pages locally (#4412)

Author: marie-bnl

content/github/working-with-github-pages/testing-your-github-pages-site-locally-with-jekyll.md

@@ -30,6 +30,7 @@ Before you can use Jekyll to test a site, you must:
 
 {% data reusables.command_line.open_the_multi_os_terminal %}
 {% data reusables.pages.navigate-publishing-source %}
+3. Run `bundle install`.
 3. Run your Jekyll site locally.
   ```shell
   $ bundle exec jekyll serve