Fix typo in selectors-3 (#12028)

Author: Psychpsyo

selectors-3/Overview.html

@@ -1541,8 +1541,8 @@ <h3 id=id-selectors><span class=secno>6.5. </span>ID selectors</h3>
   <p>An ID-typed attribute of a document language allows authors to assign an
    identifier to one element instance in the document tree. An ID selector
    contains a &quot;number sign&quot; (U+0023, <code>#</code>) immediately
-   followed by the ID value, which must be an CSS <a
-   href="https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">identifiers</a>.
+   followed by the ID value, which must be a CSS <a
+   href="https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">identifier</a>.
    An ID selector represents an element instance that has an identifier that
    matches the identifier in the ID selector.