input: Write a python function to replace a character in a string with another character.
  assert replace_char('Hello World!', 'l', 't') == 'Hetto Wortd!'
  assert replace_char('Python Programming', 'P', 'J') == 'Jython Jrogramming'
  assert replace_char('Data Science', 'a', 'o') == 'Doto Science'
output: def replace_char(s: str, old: str, new: str) -> str:
      return s.replace(old, new)