[css-shadow-parts][editorial] Switch markup in preparation for a future Bikeshed update.

Author: tabatkins

css-shadow-parts-1/Overview.bs

@@ -100,7 +100,7 @@ and allows for better encapsulation/API surface.
 
 It's important to note that ''::part()''
 offers <em>absolutely zero new theoretical power</em>.
-It is not a rehash of the ''>>>'' combinator,
+It is not a rehash of the <css>>>></css> combinator,
 it is simply a more convenient and consistent syntax
 for something authors can already do with <a>custom properties</a>.
 By separating out the explicitly "published" parts of an element