input: Write a python function to replace all spaces in a string with asterisks.
  assert replace_spaces("This is a sample string") == "This*is*a*sample*string"
  assert replace_spaces("Hello World") == "Hello*World"
  assert replace_spaces("Replace spaces with stars") == "Replace*spaces*with*stars"
output: def replace_spaces(s):
      return s.replace(' ', '*')