input: Write a function to generate a random string of a specified length using lowercase letters only. The function should return the generated string.
  assert random_string(6)=='abcdef'
  assert random_string(6)=='zyxwvu'
  assert random_string(6)=='klmnop'
  assert random_string(6)=='qrstuv'
  assert random_string(6)=='hijklm'
output: import random
  import string
  
  def random_string(length):
      return ''.join(random.choices(string.ascii_lowercase, k=length))