input: Write a python function to replace a substring in a given string with another substring.
  assert replace_string("Python", "JavaScript", "I love Python programming language") == "I love JavaScript programming language"
  assert replace_string("love", "hate", "I love Python programming language") == "I hate Python programming language"
  assert replace_string("language", "framework", "I love Python programming language") == "I love Python programming framework"
output: def replace_string(old_substring, new_substring, original_string):
      return original_string.replace(old_substring, new_substring)