Fix README URL

Author: sfackler

README.md

@@ -1,7 +1,7 @@
 # Rust-Postgres
 A native PostgreSQL driver for Rust.
 
-Documentation is available at https://sfackler.github.io/rust-postgres/doc/postgres
+Documentation is available at https://sfackler.github.io/rust-postgres/doc/master/postgres
 
 [![Build Status](https://travis-ci.org/sfackler/rust-postgres.png?branch=master)](https://travis-ci.org/sfackler/rust-postgres) [![Latest Version](https://img.shields.io/crates/v/postgres.svg)](https://crates.io/crates/postgres)