input: Write a python function to replace multiple spaces with a single space in a string.
  assert remove_extra_spaces("Hello,  world    !  How are you?") == "Hello, world! How are you?"
  assert remove_extra_spaces("This   is   a test.") == "This is a test."
  assert remove_extra_spaces("  Extra   spaces    at   start.") == " Extra spaces at start."
  assert remove_extra_spaces("NoExtraSpacesHere") == "NoExtraSpacesHere"
output: def remove_extra_spaces(s):
      return ' '.join(s.split())