input: Write a python function to alter and return a given string by replacing "!" with "?".
  assert alter_string("Python is fun!") == "Python is fun?"
  assert alter_string("Hello World!") == "Hello World?"
  assert alter_string("No exclamation marks here.") == "No exclamation marks here."
output: def alter_string(s: str) -> str:
      return s.replace('!', '?')