input: Write a function to perform linear search in a list and return the index of the target value.
  assert linear_search([1, 2, 3, 4, 5, 6], 6) == 5
  assert linear_search([10, 20, 30, 40, 50], 30) == 2
  assert linear_search([5, 7, 9, 11, 13], 8) == -1
output: def linear_search(arr, target):
      for index, value in enumerate(arr):
          if value == target:
              return index
      return -1