input: Write a python function to replace all occurrences of a given letter in a string with another.
  assert replace_letter("fortnite", "t", "l") == "forlnile"
  assert replace_letter("hello world", "o", "a") == "hella warld"
  assert replace_letter("mississippi", "s", "x") == "mixxixxippi"
output: def replace_letter(s: str, old: str, new: str) -> str:
      return s.replace(old, new)