[css-ui-4] switch from using :focus to :focus-visible in the example on outlines

[bkardell]

[css-ui-4] closes #12538

Note, given that I changed this to use 'state' I also changed active to mention the state. If this is problematic I guess it could also be something like

For example, to draw a thick black line around elements when the UA
would draw a native focus outline, and a thick red line when it is active,
the following rules can be used:

But to me, what I sent is clearer.

Copy the above line into the Title and replace with the relevant details. Fill in any additional details here. See https://github.com/w3c/csswg-drafts/blob/master/CONTRIBUTING.md for more info.