[css-display] Re-order the <display-listitem> grammar to put 'list-item' last. Fixes #1621.

Author: tabatkins

css-display-3/Overview.bs

@@ -164,7 +164,7 @@ Box Layout Modes: the 'display' property</h2>
 	<pre class='prod'>
 	<dfn>&lt;display-outside></dfn>  = block | inline | run-in ;
 	<dfn>&lt;display-inside></dfn>   = flow | flow-root | table | flex | grid | ruby ;
-	<dfn>&lt;display-listitem></dfn> = list-item && <<display-outside>>? && [ flow | flow-root ]? ;
+	<dfn>&lt;display-listitem></dfn> = <<display-outside>>? && [ flow | flow-root ]? && list-item;
 	<dfn>&lt;display-internal></dfn> = table-row-group | table-header-group |
 	                     table-footer-group | table-row | table-cell |
 	                     table-column-group | table-column | table-caption |