Fill in RFC number in Constrained Naked

Author: npmccallum

text/2972-constrained-naked.md

@@ -1,6 +1,6 @@
 - Feature Name: `constrained_naked`
 - Start Date: 2020-08-06
-- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- RFC PR: [rust-lang/rfcs#2972](https://github.com/rust-lang/rfcs/pull/2972)
 - Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
 
 # Summary