docs: fix install block style

README.md

@@ -14,7 +14,7 @@ This is a [Node.js](https://nodejs.org/en/) module available through the
 [npm registry](https://www.npmjs.com/). Installation is done using the
 [`npm install` command](https://docs.npmjs.com/getting-started/installing-npm-packages-locally):
 
-```bash
+```sh
 $ npm install cookie-session
 ```