Fix typo in Chapter 09

Author: mashabow

ch09.md

@@ -313,7 +313,7 @@ const upload = (filename, callback) => {
     throw new Error('You need a filename!');
   } else {
     readFile(filename, (errF, contents) => {
-      if (errF) throw err;
+      if (errF) throw errF;
       httpPost(contents, (errH, json) => {
         if (errH) throw errH;
         callback(json);