output MDL-25445 Corrected standard_end_of_body_html, returns rather …

…than echoes now as suggested by docs and use. Kudos Urs for spotting.

lib/outputrenderers.php

@@ -379,7 +379,7 @@ public function standard_end_of_body_html() {
         // This function is normally called from a layout.php file in {@link header()}
         // but some of the content won't be known until later, so we return a placeholder
         // for now. This will be replaced with the real content in {@link footer()}.
-        echo self::END_HTML_TOKEN;
+        return self::END_HTML_TOKEN;
     }
 
     /**