Write a python program that takes a list of strings and returns a sorted copy of the list.
l1 = ['cat', 'dog', 'bird', 'elephant'
def sort_list(l):
    l.sort()
    return l

sorted_list = sort_list(['cat', 'dog', 'bird', 'elephant'])

print(sorted_list) # Output: ['bird', 'cat', 'dog', 'elephant']