input: Write a function to replace all occurrences of a character in a string with another character.
  assert replaceChar("hello world", "l", "a") == "heaaao worad"
  assert replaceChar("banana", "a", "o") == "bonono"
  assert replaceChar("mississippi", "i", "u") == "mussussuppu"
output: def replaceChar(s: str, old: str, new: str) -> str:
      return s.replace(old, new)