input: Write a python function to add one to each character in a string.
  assert add_one_to_each_char("abc") == "bcd"
  assert add_one_to_each_char("hello") == "ifmmp"
  assert add_one_to_each_char("xyz") == "yza"
output: def add_one_to_each_char(s):
      return ''.join(chr(ord(c) + 1) if c != 'z' else 'a' for c in s)