input: Write a python function to generate a random string of specified length using ascii_letters from the string module.
  assert len(generateRandomString(10)) == 10
  assert len(generateRandomString(5)) == 5
  assert isinstance(generateRandomString(8), str)
output: import random
  import string
  
  def generateRandomString(length):
      return ''.join(random.choice(string.ascii_letters) for _ in range(length))