input: Write a python function to reverse a substring within a given string.
  assert reverse_substring("Hello world", 0, 4) == "olleH world"
  assert reverse_substring("abcdefg", 2, 5) == "abfedcg"
  assert reverse_substring("The quick brown fox", 10, 14) == "The quick worbn fox"
output: def reverse_substring(s: str, start: int, end: int) -> str:
      return s[:start] + s[start:end+1][::-1] + s[end+1:]