input: Write a function to extract a substring between two given keywords from a string.
  assert extract_substring("Hello, my name is John and I love programming.", "name is ", " and") == "John"
  assert extract_substring("Find the substring between two words in this sentence.", "substring ", " words") == "between two"
  assert extract_substring("Python programming is fun!", "Python ", " is") == "programming"
output: def extract_substring(s, start, end):
      start_index = s.find(start) + len(start)
      end_index = s.find(end, start_index)
      return s[start_index:end_index]