Consistent usage in 4-application-development.md

[adnanvaldes]

In the Epilogue of Part 10, Section 4, the dependency injection example uses storage_service to illustrate the ability to maintain a user interface even when storage locations change.

However, in the example it shows:

class PhoneBookApplication:
    def __init__(self, filename):
        self.__phonebook = PhoneBook()
        self.__filename = filename

    # the rest of the user interface

# use a different filename
storage_service = FileHandler("new_phonebook.txt")

The PhoneBookApplication class constructor should continue to use storage_service with the filename change only being applied in the name passed to FileHandler.