input: Write a python function to convert a list of strings to lowercase.
  assert to_lower(['HELLO', 'FRIENDS', 'WORLD']) == ['hello', 'friends', 'world']
  assert to_lower(['Python', 'Java', 'C++']) == ['python', 'java', 'c++']
  assert to_lower(['LOWER', 'CASE', 'TEST']) == ['lower', 'case', 'test']
output: def to_lower(lst):
      return [s.lower() for s in lst]