Add RFC PR link

Author: oli-obk

text/3243-packages-as-optional-namespaces.md

@@ -1,6 +1,6 @@
 - Feature Name: `packages_as_namespaces`
 - Start Date: (fill me in with today's date, 2022-03-09)
-- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- RFC PR: [rust-lang/rfcs#3243](https://github.com/rust-lang/rfcs/pull/3243)
 - Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
 
 # Summary