input: Write a python function to perform a linear search in a list.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 40) == 3
  assert linear_search([10, 20, 30, 40, 50], 60) == -1
output: def linear_search(lst, target):
      for index, value in enumerate(lst):
          if value == target:
              return index
      return -1