docs: fix broken link to service users section in link-source.md

The 'Service users for team autobuilds' section lives in setup.md,
not in _index.md. Point the 'create a service account' link to
setup.md#service-users-for-team-autobuilds so it resolves correctly.

Author: nielskaspers

content/manuals/docker-hub/repos/manage/builds/link-source.md

@@ -21,7 +21,7 @@ code service to Docker Hub so that it can access your source code
 repositories. You can configure this link for user accounts or
 organizations.
 
-If you are linking a source code provider to create autobuilds for a team, follow the instructions to [create a service account](index.md#service-users-for-team-autobuilds) for the team before linking the account as described below.
+If you are linking a source code provider to create autobuilds for a team, follow the instructions to [create a service account](setup.md#service-users-for-team-autobuilds) for the team before linking the account as described below.
 
 ## Link to a GitHub user account