Fix validate_initial_digits of credit_card_validator.py (#5600)

* Fix validate_initial_digits of credit_card_validator.py

@Bhargavishnu I think that I broke the logic of validate_initial_digits which should require that credit_card_number[0] is 3 before checking that credit_card_number[1] is 4, 5, or 7.  Please verify the new changes and the new test cases to make sure that this is correct.  Thanks!

* updating DIRECTORY.md

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