Clarify that "image-label" and "multiscales" are siblings

[will-moore]

I found it spec slightly confusing, based on this wording:

3.4. "image-label" metadata Groups containing the image-label dictionary represent an image segmentation in which each unique pixel value represents a separate segmented object. image-label groups MUST also contain multiscales metadata

Since the "multiscales" object is not shown in the example, I could imagine that image/labels/0/.zattrs could contain "image-label" that MUST contain "multiscales" like:

{
    "image-label": {
        "multiscales": [],
        "colors": [],
    }
}

The 'tree' outline doesn't clarify this either - doesn't mention that .zattrs here MUST contain "multiscales"

├── .zattrs   # Metadata of the related image and as well as display information under the "image-label" key.

In fact, image/labels/0/.zattrs should be: { "multiscales": [], "image-label": { "colors": [], } }

cc @sbesson