input: Write a python function to perform a linear search on a list.
  assert linear_search([10, 50, 30, 20, 11], 20)==3
  assert linear_search([1, 2, 3, 4, 5], 3)==2
  assert linear_search([100, 200, 300, 400, 500], 600)==-1
output: def linear_search(arr, target):
      for i in range(len(arr)):
          if arr[i] == target:
              return i
      return -1