README: fix doc link

Author: andelf

README.md

@@ -4,7 +4,7 @@ rust-postgis
 [![Build Status](https://travis-ci.org/andelf/rust-postgis.svg?branch=master)](https://travis-ci.org/andelf/rust-postgis)
 [![Crates.io](https://meritbadge.herokuapp.com/postgis)](https://crates.io/crates/postgis)
 
-[Documentation](http://www.rust-ci.org/andelf/rust-postgis/doc/postgis/)
+[Documentation](https://docs.rs/postgis/)
 
 An extension to rust-postgres, adds support for PostGIS.