Change comment type to prevent empty CSS selector

Sass will emit an empty CSS selector when an orphan rule contains a `/* */` comment block

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Let me know if you want a built version with the PR, but it will remove this dead CSS rule https://github.com/dimsemenov/Magnific-Popup/blob/844ffea1c9e817b69eb3e5a46d4c2402cb835685/dist/magnific-popup.css#L312-L313