changle import style on first example (#17)

Author: slonkazoid

README.md

@@ -20,7 +20,7 @@ npm install --save keyv keyv-file
 ### Using with keyv
 ```js
 const Keyv = require('keyv')
-const KeyvFile = require('keyv-file').KeyvFile
+const { KeyvFile } = require('keyv-file')
 
 const keyv = new Keyv({
   store: new KeyvFile()