Update association_basics.md for belongs_to and has_one [ci skip].

This is the Reference section, and I believe I found an error. Or at least, an error by omission. The original text states that belongs_to creates a one-to-one relationship. But IMO this is not correct. Taken alone, it creates a many-to-one relationship with another model. It is additionally used to create a one-to-one when the other model is given has_one. I've updated the guide to state this.