Fixed broken link to implementation of JSON Server provider

[gustavorps]

Current documentation link is broken: https://github.com/okami101/vuetify-admin/blob/master/packages/admin/src/providers/data/json-server.js

vuetify-admin/docs/guide/tutorial.md

Line 72 in 5c97b6f

As you will see, the JSON Server data provider is a simple JS object that does all the magic behind the scenes. It translates the particular [JSON Server API format](https://github.com/typicode/json-server#routes) into simple methods that Vuetify Admin can understand. For that each provider must follow a specific implementation as [explained here](data-providers.md). You can find the source code of JSON Server data provider [here](https://github.com/okami101/vuetify-admin/blob/master/packages/admin/src/providers/data/json-server.js)