replace #cf_descriptor{} with cf_descriptor() type

Currently rocksdb.erl contains incorrect type specs i.e. it says cf_descriptor must be record, while in reality it doesn't work that way, if you try to open_with_cf with list of cf_descriptor records it fails. Also, readme uses list of tuples {string(), cf_options{}}. This pull-requests corrects this inconsistency by defining cf_descriptor to be type for aforementioned record thus making Dialyzer (and developer) happy.