input: Write a python function to perform linear search.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 35) == -1
  assert linear_search([7, 5, 9, 1, 3], 9) == 2
output: def linear_search(arr, target):
      for index, value in enumerate(arr):
          if value == target:
              return index
      return -1