Fix the link (#78)

Author: f3ath

README.md

@@ -1,6 +1,6 @@
 # [JSON API](http://jsonapi.org) spec implemented in PHP 7. Immutable
 
-**This is v2 of the implementation. For v1 click [here](/json-api-php/json-api/tree/v2).**
+**This is v2 of the implementation. For v1 click [here](./tree/v1).**
 
 The goal of this library is to ensure strict validity of JSON API documents being produced.