Fix Oxford comma in join us page description (#8661)

Thx @joshjyu and reviewers!

The pull request was done with the correct branch.

There's a linked issue. I read/understood it.

Your edit in the Files Changed tab looks good.

I checked the change locally in Docker. All good.

Author: joshjyu

pages/join-us.html

@@ -12,7 +12,7 @@
       <h1 class="title1">Join Us</h1>
       <p>
         We bring together civic-minded volunteers to build digital products,
-        programs and services with community partners and local government to
+        programs, and services with community partners and local government to
         address issues in our LA region and to share these effective processes,
         practices, and code with the larger civic tech community.
       </p>