[css-overflow] 'overflow' property values aren't properly linkified in definition of scrollbar-gutter 'stable' value

[dholbert]

The definition of "scrollbar-gutter: stable" (at https://drafts.csswg.org/css-overflow-4/#scrollbar-gutter-property ) has some keyword names that look they're intended to linkified to definitions but aren't currently.

In particular:

• there's a reference to "overflow/scroll" and "overflow/auto" (with property-name and slash) which look like they're not getting recognized/escaped properly.
• Also, there's a reference to "hidden" which is blue but not linkified, possibly because its quote marks are mismatched (looks like a backtick before and basic vertical-bar single-quote after, at least in the rendered output).

See screenshot:

CC @frivoal who's been working on this text I think.

[frivoal]

Works now.