Cleaned code

BookHive/Validators/CustomValidators.cs

@@ -20,6 +20,7 @@ public static class CustomValidators
                     var digit = isbn[i] - '0';
                     sum += (i % 2 == 0) ? digit : digit * 3;
                 }
+
                 var checkDigit = (10 - (sum % 10)) % 10;
                 return checkDigit == (isbn[12] - '0');
             })