input: Write a python function to replace a character in a string with another character.
  assert replace_char("This is an example", "s", "z") == "Thiz iz an example"
  assert replace_char("hello world", "o", "0") == "hell0 w0rld"
  assert replace_char("abcdef", "a", "z") == "zbcdef"
  assert replace_char("testing", "t", "T") == "TesTing"
output: def replace_char(s: str, old: str, new: str) -> str:
      return s.replace(old, new)