input: Write a python function to perform a linear search on a list.
  assert linear_search([2, 3, 5, 7, 11], 5) == 2
  assert linear_search([1, 2, 3, 4, 5], 1) == 0
  assert linear_search([1, 2, 3, 4, 5], 6) == -1
output: def linear_search(lst, target):
      for index, value in enumerate(lst):
          if value == target:
              return index
      return -1