Update README.md

Jiang-Xuan · web-flow · commit f111e8067047 · 2017-03-24T18:10:15.000+08:00
diff --git a/README.md b/README.md
@@ -41,7 +41,7 @@ NOTE: from log4js 0.5 onwards you'll need to explicitly enable replacement of no
 
 ## installation
 
-```shell
+```bash
 npm install log4js
 ```
 
