[css-gcpm-3] fix errors in example

Author: dauwhe

css-gcpm-3/Overview.bs

@@ -783,11 +783,11 @@ Consider the following HTML:
 And CSS:
 
 <pre>
-div.Chapter {
+div.chapter {
 page: body;
 }
 
-div.broadside {
+table.broadside {
 page: broadside;
 }