fix: changed account property from BankAccount to CheckingAccount in Check.java

Author: mercedes-mathews

lesson_17/bank/bank_app/src/main/java/com/codedifferently/lesson17/bank/Check.java

@@ -7,7 +7,7 @@ public class Check {
 
   private final String checkNumber;
   private final double amount;
-  private final BankAccount account;
+  private final CheckingAccount account;
   private boolean isVoided = false;
 
   /**