[css-shadow-parts-1] Fix innerIdent/outerIdent ordering

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -195,11 +195,11 @@ The partmap attribute is parsed as a comma-separated list of part mappings.
 Each part mapping is one of:
 
 <dl class=switch>
-    : <code>outerIdent => innerIdent</code>
+    : <code>innerIdent outerIdent</code>
     :: this adds «[ outerIdent → innerIdent ]» to el's <a>part name map</a>.
 
     : <code>ident</code>
-    :: Is equivalent to <code>ident => ident</code>.
+    :: Is shorthand for <code>ident ident</code>.
 
     : anything else
     :: Ignored for error-recovery / future compat.