fix incorrect syntax highlighting in readme

PR-URL: #164
Credit: @amilajack
Close: #164
Reviewed-by: @isaacs

README.md

@@ -18,7 +18,7 @@ performance, by causing the cache to have to do more work.  See the
 
 ## Installation
 
-```js
+```bash
 npm install lru-cache --save
 ```