Fix tracking issue in defaulted type params RFC

It previously pointed to one closed as a duplicate

Author: kamalmarhubi

text/0213-defaulted-type-params.md

@@ -1,6 +1,6 @@
 - Start Date: 2015-02-04
 - RFC PR: [rust-lang/rfcs#213](https://github.com/rust-lang/rfcs/pull/213)
-- Rust Issue: [rust-lang/rust#21939](https://github.com/rust-lang/rust/issues/21939)
+- Rust Issue: [rust-lang/rust#27336](https://github.com/rust-lang/rust/issues/27336)
 
 # Summary