input: Write a python function to replace a substring in a string with another substring.
  assert replace_substring("Hello, world!", "world", "Python") == "Hello, Python!"
  assert replace_substring("I love cats", "cats", "dogs") == "I love dogs"
  assert replace_substring("Good morning", "morning", "night") == "Good night"
output: def replace_substring(original, to_replace, replacement):
      return original.replace(to_replace, replacement)