void encrypt(const char* plaintext, int n, char* ciphertext)
// Purpose: 1) takes the string in plaintext and
//             copies it to the string ciphertext
//          2) encrypts the ciphertext with the 
//             Ceaser cipher with shift n 



// Note: the Ceaser cipher simply takes each character
//       in the plaintext and replaces with the letter 
//       shifted n places to the right in the alphabet. 
//       The alphabet for us will be the ASCII values 
//       Thus, the encryption basically takes
//       each character in the input plaintext string
//       and replaces it with the character with ASCII
//       value n greater than it.