[css-shadow-parts-1] Fix lingering reference to 'shadow parts map'.

Author: tabatkins

css-shadow-parts-1/Overview.bs

@@ -104,7 +104,7 @@ It is not a rehash of the ''>>>'' 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
-(the <a>shadow part map</a>
+(the <a>part element map</a>)
 from the sub-parts that it merely happens to contain,
 it also helps with encapsulation,
 as authors can use ''::part()'' without fear of accidental over-styling.