DOC: Fixed incorrect assert_array_almost_equal_nulp documentation

The `max()` function previously used does not work with two array-like.
`maximum()` does, and is what is essentially used in the code
(`ref = nulp * np.spacing(np.where(ax > ay, ax, ay))`).
Merge pull request #5374 from lebigot/patch-1

DOC: Fixed incorrect assert_array_almost_equal_nulp documentation