input: Write a python function to slice a string from a given start index to an end index.
  assert slicing("hello world", 0, 5) == "hello"
  assert slicing("python programming", 7, 18) == "programming"
  assert slicing("data science", 5, 12) == "science"
output: def slicing(s, start, end):
      return s[start:end]