input: Write a python function to replace all occurrences of a character x with another character y in a given string.
  assert replaceXWithY("xx123xx456xx", "x", "y") == "yy123yy456yy"
  assert replaceXWithY("hello world", "o", "a") == "hella warld"
  assert replaceXWithY("abcdefg", "a", "z") == "zbcdefg"
output: def replaceXWithY(s: str, x: str, y: str) -> str:
      return s.replace(x, y)