input: Write a python function to replace a substring within a given string with a new substring.
  assert replace_substring("hello world", "world", "Earth") == "hello Earth"
  assert replace_substring("good morning", "morning", "evening") == "good evening"
  assert replace_substring("I love cats", "cats", "dogs") == "I love dogs"
output: def replace_substring(original_string, old_substring, new_substring):
      return original_string.replace(old_substring, new_substring)