Fix dingus link when double clicking Markdown code (#535)

Prior to this commit, the link opened is always `/dingus/?text=` (no text),
because `$(this)` is `code.language-markdown`, so `.find`ing `code` in its
descendants turns up nothing.

tools/template.html

@@ -108,7 +108,7 @@
     });
   });
   $$("code.language-markdown").dblclick(function(e) { window.open('/dingus/?text=' +
-      encodeURIComponent($$(this).find('code').text()));
+      encodeURIComponent($$(this).text()));
   });
 });
 </script>