input: Write a python function to replace spaces with underscores in a string.
  assert replace_space("The quick brown fox") == "The_quick_brown_fox"
  assert replace_space("Hello World") == "Hello_World"
  assert replace_space("Python Programming Language") == "Python_Programming_Language"
output: def replace_space(s: str) -> str:
      return s.replace(' ', '_')