Revert "Correct pull request URL in RFC rust-lang#1444"

Author: petertodd

text/1444-union.md

@@ -1,6 +1,6 @@
 - Feature Name: `union`
 - Start Date: 2015-12-29
-- RFC PR: https://github.com/rust-lang/rfcs/pull/1444
+- RFC PR: https://github.com/rust-lang/rfcs/pulls/1444
 - Rust Issue: https://github.com/rust-lang/rust/issues/32836
 
 # Summary