input: Write a python function to check if one string is a rotation of another string.
  assert is_rotation("waterbottle", "erbottlewat") == True
  assert is_rotation("hello", "oellh") == False
  assert is_rotation("abcd", "cdab") == True
  assert is_rotation("rotation", "rotation") == True
output: def is_rotation(s1, s2):
      if len(s1) != len(s2):
          return False
      return s2 in s1 + s1