Fix documentation of next.jdbc/execute-batch!

Searching for documentation for `next.jdbc/execute-batch!`, I tried to execute the "equivalent to" examples of the `next.jdbc.sql/insert-multi!` documentation, but encountered: ``` java.lang.ClassCastException: class clojure.lang.PersistentArrayMap cannot be cast to class java.sql.PreparedStatement ``` Fixes: 3ed1f4b99c975b556327d1abb9b387db8a6d93a6