link rfc 3086 to its tracking issue

Author: nikomatsakis

text/3086-macro-metavar-expr.md

@@ -1,7 +1,7 @@
 - Feature Name: `macro_metavar_expr`
 - Start Date: 2021-01-23
-- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
-- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+- RFC PR: [rust-lang/rfcs#3086](https://github.com/rust-lang/rfcs/pull/3086)
+- Rust Issue: [rust-lang/rust#83527](https://github.com/rust-lang/rust/issues/83527)
 
 # Summary
 [summary]: #summary