Method that accepts a string, and returns true if it is in the form of a phone number.
Assume that any integer from 0-9 in any of the spots will produce a valid phone number.
Only worry about the following format: (123) 456-7890 (don't forget the space after the close parentheses)
Examples:
validPhoneNumber("(123) 456-7890") => returns true validPhoneNumber("(1111)555 2345") => returns false validPhoneNumber("(098) 123 4567") => returns false