fix(shadcn): add missing data-selected selector for boolean attribute

[oleksandrkovaliuk]

Why

Fixes overwritten base-ui data-selected styles by adding(accidentally missed) selector to the data-selected custom variant.

Context

#11038

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[oleksandrkovaliuk]

Made a tiny change by adding a selector that was likely accidentally missed.
The workaround for now is to override it in your own index.css, so it's not that urgent.