input: Write a python function to replace all occurrences of a substring with another substring in a given string.
  assert replace_substring("hello world", "world", "there") == "hello there"
  assert replace_substring("hello hello hello", "hello", "hi") == "hi hi hi"
  assert replace_substring("abcabcabc", "abc", "xyz") == "xyzxyzxyz"
  assert replace_substring("nothing to replace here", "test", "replace") == "nothing to replace here"
output: def replace_substring(original_string, old_substring, new_substring):
      return original_string.replace(old_substring, new_substring)