[css-nesting] fixes missing closing div on an example

Author: Adam Argyle

css-nesting-1/Overview.bs

@@ -352,6 +352,7 @@ The Nesting At-Rule: ''@nest'' {#at-nest}
 		/* Invalid because not all selectors in the list
 		   contain a nesting selector */
 		</pre>
+	</div>
 
 Mixing Nesting Rules and Declarations {#mixing}
 -----------------------------------------------