Correct Password Grant Callback for UserID Storage

This is an update to the suggested callback for password grants.

After testing I found that all of my applications were authorizing to user with id of one when using password grant.

I was looking around to figure out what was going on and found that the PasswordGrant class expects the callback to return `false` when not authorized or the user id.

Since the old callback was sending back true, Laravel's DB checker was casting this to `1` and all my requests where then showing the session's user as the user where `id` = `1`.

I'm also going to look in the wiki and update the docs for this change.