Use doc.rs URL for documentation link in README

Author: mattnenterprise

README.md

@@ -6,7 +6,7 @@ FTP client for Rust
 [![Build Status](https://travis-ci.org/mattnenterprise/rust-ftp.svg)](https://travis-ci.org/mattnenterprise/rust-ftp)
 [![crates.io](http://meritbadge.herokuapp.com/ftp)](https://crates.io/crates/ftp)
 
-[Documentation](http://mattnenterprise.github.io/rust-ftp)
+[Documentation](https://docs.rs/ftp/)
 
 ## Installation