Added clarity for Deny Callbacks in posts.js

[JeremyIglehart]

I'm new to Meteor Development, and OrionJS for that matter.

This little problem had me running around for a couple hours.

Perhaps we could solve this for future readers by documenting that we have to remove this Deny Callback in order to make any updates from the admin panel.

Like I said before, I'm new to Meteor and OrionJS - so I might not be right here. If I'm making a mistake please let me know.

Otherwise, maybe these contributions to the tutorial will be welcome.