update readme

Author: wangdong

README.md

@@ -38,7 +38,7 @@ Include the following script tags in your HTML:
 <script src="../bower_components/async/dist/async.js"></script>
 <script src="../bower_components/axios/dist/axios.min.js"></script>
 <script src="../bower_components/bignumber.js/bignumber.min.js"></script>
-<script src="../bower_components/axios/lib/bn.js"></script>
+<script src="../node_modules/bn.js/lib/bn.js"></script>
 <script src="../bower_components/lodash/dist/lodash.min.js"></script>
 <script src="../bower_components/loopring.js/dist/loopring.min.js"></script>
 ```