input: Write a function to reverse an array of numbers.
  assert reverse_array([1, 2, 3, 4, 5]) == [5, 4, 3, 2, 1]
  assert reverse_array([10, 20, 30, 40]) == [40, 30, 20, 10]
  assert reverse_array([7, 14, 21]) == [21, 14, 7]
output: def reverse_array(arr):
      return arr[::-1]