[css-spec-selectors-4] Corrected a typo (replacing a sentence-final comma with period)

[yg-i]

In the first paragraph of this section (https://drafts.csswg.org/selectors/#scoping), changed the comma after 'of the document' to period

Some host applications may choose to scope selectors to a particular subtree or fragment of the document, The root of the scoping subtree is called the scoping root.