Update 2789 links.

Author: ehuss

text/0000-sparse-index.md

@@ -1,7 +1,7 @@
 - Feature Name: sparse_index
 - Start Date: 2019-10-18
-- 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#2789](https://github.com/rust-lang/rfcs/pull/2789)
+- Tracking Issue: [rust-lang/rust#9069](https://github.com/rust-lang/cargo/issues/9069)
 
 # Summary
 [summary]: #summary