Update README.md

Author: macrozheng

README.md

@@ -23,7 +23,7 @@
 
 <p align="left">
 <a href="https://e.coding.net/?utm_source=macrozheng&utm_medium=banner&utm_campaign=march2019" target="_blank">
- <img src="http://macro-oss.oss-cn-shenzhen.aliyuncs.com/mall/banner/coding.jpg" width=""/>
+ <img src="http://macro-oss.oss-cn-shenzhen.aliyuncs.com/mall/banner/coding.png" width=""/>
  </a>
 </p>