Update html_root_url for 0.12.0 release

Author: brson

lib.rs

@@ -21,7 +21,7 @@
 #![crate_type = "rlib"]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
-       html_root_url = "http://doc.rust-lang.org/master/",
+       html_root_url = "http://doc.rust-lang.org/0.12.0/",
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![feature(macro_rules, phase, globs)]