Sidebar permalink panel: improve the URL field label

[afercia]

Splitting this out from #11874.

I'd tend to think the input field label URL is a bit confusing, as users might think they have to fill the field with a complete URL starting with http(s).

I've read #11858 (comment) and it makes perfectly sense to avoid technical terms. However, the label could be improved a bit. Also noting Squarespace does use some form of "URL" label but it also expands it adding "slug":

Other mentioned platforms use some form of "URL" as well, but at least they make it visually clear it's about the last part of the URL:

Personally, I have no strong opinions on which the best wording would be, but I'd suggest to consider a more meaningful one.

[afercia]

Not sure if it can help, but worth considering the explanation of what a "permalink" is in WordPress in the Help page linked from the Classic Editor help tab:
https://codex.wordpress.org/Posts_Add_New_Screen

[timwright12]

@afercia I updated it to "URL Slug" just so we have an open PR. Happy to update it, but I think that actually works pretty well.