fix docs link

Author: Dónal

README.md

@@ -1,5 +1,5 @@
 # grails-hibernate-filter
 
-This repository contains the source code for the Grails 1.X and 2.X versions of the Hibernate Filter plugin. Documentation is [available here](User guide).
+This repository contains the source code for the Grails 1.X and 2.X versions of the Hibernate Filter plugin. Documentation is [available here](https://grails.org/plugin/hibernate-filter).
 
 Information about the Grails 3.X version of this plugin is [available here](https://bintray.com/piotrchowaniec/grails-plugins/grails-hibernate-filter).