Updating Corporate Marketing Handbook to swap mentions of retired Slack...

Updating Corporate Marketing Handbook to swap mentions of retired Slack channels to new social-media channel

source/handbook/marketing/corporate-marketing/index.html.md

@@ -681,4 +681,4 @@ In the event that a media member, editor, or publisher offers a draft or preview
 #### Social media 
-Please consult the [Social Marketing Handbook](/handbook/marketing/corporate-marketing/social-marketing/). If you are contacted on a social media platform and asked to share/retweet or provide commentary as a spokesperson of GitLab, feel welcome to provide detail of the opportunity to the social team in the `#social` Slack channel. 
+Please consult the [Social Marketing Handbook](/handbook/marketing/corporate-marketing/social-marketing/). If you are contacted on a social media platform and asked to share/retweet or provide commentary as a spokesperson of GitLab, feel welcome to provide detail of the opportunity to the social team in the `#social-media` Slack channel.