Update Slack Invite URL

Author: mzagaja

_config.yml

@@ -28,7 +28,7 @@ meetup_url: https://meetup.com/Code-For-Boston
 blog_url: https://medium.com/@CodeForBoston
 events_url: https://cal.codeforboston.org
 zoom_url: https://codeforamerica.zoom.us/meeting/register/tJYoc-murzMtG9wWCo8JE4GlkByYio1VnuDX
-slack_url: https://communityinviter.com/apps/cfb-public/code-for-boston-slack-invite
+slack_url: https://communityinviter.com/apps/cfb-public/default-badge
 youtube_url: https://www.youtube.com/c/codeforboston
 facebook_url: https://www.facebook.com/codeforboston/
 instagram_url: https://www.instagram.com/codeforboston/