Edited explanation of associate method

Just a minor suggested edit to the explanation of the belongsTo associate() method to make it clearer that the method needs to be chained to the model with the belongsTo relationship - and used the $user -> $phone relationship example from further up in the docs - the Account/User example was not quite as clear as to which was the belongsTo and which was the hasOne Model. 

The existing example may not be quite as clear as possible given the likes of this question - http://stackoverflow.com/questions/19868838/call-to-undefined-method-illuminate-database-query-builderassociate/19993027#19993027 - but that may just be me - and I may not be 100% correct in my understanding - so no major if you think it should stay as is.