input: Write a python function to replace each letter in a string with the next letter in the alphabet, and replace 'z' with 'a'.
  assert replace_letter('abc')=='bcd'
  assert replace_letter('xyz')=='yza'
  assert replace_letter('hello')=='ifmmp'
output: def replace_letter(s):
      result = ''
      for char in s:
          if char == 'z':
              result += 'a'
          else:
              result += chr(ord(char) + 1)
      return result