bug#8495: 24.0.50: elisp info typo

[Alp Aker]

The info node `(elisp)Image Descriptors' states that:

  Laplace edge-detection currently uses a matrix of
       (1  0  0
        0  0  0
        9  9 -1)

but the matrix is defined in image.c as:

  static int laplace_matrix[9] = {
     /* x - 1     x       x + 1  */
          1,      0,      0,      /* y - 1 */
          0,      0,      0,      /* y     */
          0,      0,      -1      /* y + 1 */
  };