user guide: fill in Default Construction Variables section

rb/assets/user_guide.html.erb

@@ -43,6 +43,8 @@
       }
       .code {
         padding-left: 2ex;
+        width: 116ex;
+        overflow-x: auto;
       }
       .ruby_code .normal {}
       .ruby_code .comment { color: #005; font-style: italic; }