input: Write a function to implement linear search.
  assert linear_search([1, 2, 3, 4, 5, 6], 4) == 3
  assert linear_search([10, 20, 30, 40, 50], 30) == 2
  assert linear_search([7, 8, 9, 10, 11], 12) == -1
output: def linear_search(arr, target):
      for index, value in enumerate(arr):
          if value == target:
              return index
      return -1