Fix call to img_as_ubyte

[iimog]

The function img_as_ubyte needs to be called as skimage.util.img_as_ubyte. The util part was sometimes missing.

[mkcor]

No, it's not necessary: I just checked and I could equally from skimage import img_as_ubyte and from skimage.util import img_as_ubyte. I guess the magic is happening here: https://github.com/scikit-image/scikit-image/blob/533a5b5c0852b38890d63c348117aba8055b3995/skimage/__init__.py#L152-L158

[iimog]

Nice, I did not know that. I still think it is better to call it consistently (either with or without util). I think it fits slightly better to use the util submodule as most other functions are called from submodules as well. But I'm happy to discuss the pro and cons.

[iimog]

Your comment also made me realize, that in the "Resizing an image" box, we need to also import skimage.util or import skimage. Otherwise, neither of the ways to call img_as_ubyte will work.

[mkcor]

You are right that, for educational purposes, it's probably better to keep the util submodule explicit.

[tobyhodges]

Thank you @iimog 🙌