input: Write a python function to replace spaces in a string with underscores.
  assert replace_spaces_with_underscore("Hello World") == "Hello_World"
  assert replace_spaces_with_underscore("Python is awesome") == "Python_is_awesome"
  assert replace_spaces_with_underscore("Replace all spaces") == "Replace_all_spaces"
output: def replace_spaces_with_underscore(s):
      return s.replace(" ", "_")