void decrypt(const char* ciphertext , int n, char* plaintext)
// Purpose: 1) takes the string in ciphertext and
//             copies it to the string plaintext
//          2) decrypts the ciphertext with the 
//             inverse Ceaser cipher with shift n 
//             (replaces each character with the 
//              charcater with ASCII value n less than
//              the current value).