This file is rebuilt by the App Manager from /etc/hildon-application-manager/catalogues, so it doesn't really matter if you mess it up, it does also mean the next time you make any changes to the catalogues in the App Manager it will rewrite /etc/apt/sources.list.d/hildon-application-manager.list and you'll lose any changes you make. If this file is messed up, rather than editing it yourself it is probably better to check what it says in Application Catalogues in the App Man, if something is wrong fix it, if it looks okay then make a minor change, maybe disable one of the catalogues and then it will rewrite than file.