[selectors] Put the column combinator in its heading, like the other combinators.

Author: tabatkins

selectors-4/Overview.bs

@@ -3228,7 +3228,7 @@ Grid-Structural Selectors</h2>
 	In a column-primary format, these pseudo-classes match against row associations instead.
 
 <h3 id="the-column-combinator">
-Column combinator</h3>
+Column combinator (<code>||</code>)</h3>
 
 	The <dfn export>column combinator</dfn>, which consists of two pipes (<dfn selector id="selectordef-column">||</dfn>)
 	represents the relationship of a column element