input: Write a python function to perform a linear search in a list.
  assert linear_search([5,3,7,2,1], 3) == 1
  assert linear_search([10,20,30,40,50], 40) == 3
  assert linear_search([1,2,3,4,5], 6) == -1
output: def linear_search(lst, target):
      for i in range(len(lst)):
          if lst[i] == target:
              return i
      return -1