input: Write a python function to replace a substring within a string with another substring.
  assert replace_string('This is an old_string', 'old_string', 'new_string') == 'This is an new_string'
  assert replace_string('Hello old world', 'old', 'new') == 'Hello new world'
  assert replace_string('Repeat, repeat, repeat', 'repeat', 'done') == 'Repeat, done, done'
output: def replace_string(s, old, new):
      return s.replace(old, new)