Fix link

github doesn't auto link this

Author: steveklabnik

text/2657-roadmap-2019.md

@@ -2,7 +2,7 @@
 
 - Feature Name: N/A
 - Start Date: 2019-03-07
-- RFC PR: rust-lang/rfcs#2657
+- RFC PR: [rust-lang/rfcs#2657](https://github.com/rust-lang/rfcs/pull/2657)
 - Rust Issue: N/A
 
 # Summary