docs: Fix "prerequisite:" in Omnibus docs

We've standardized on the plural, so let's fix this.

Author: amyq

doc/build/team_member_docs.md

@@ -117,7 +117,7 @@ below for details:
 
 ## Build packages for other operating systems
 
-Prerequisite:
+Prerequisites:
 
 - You must have permission to push branches to the `omnibus-gitlab` [release mirror](https://dev.gitlab.org/gitlab/omnibus-gitlab).