Fill in the tracking issue number

Author: saethlin

src/doc/src/reference/unstable.md

@@ -1673,7 +1673,7 @@ cargo-features = ["open-namespaces"]
 
 ## panic-immediate-abort
 
-* Tracking Issue: [#0000](https://github.com/rust-lang/cargo/issues/0000)
+* Tracking Issue: [#16042](https://github.com/rust-lang/cargo/issues/16042)
 
 Allow use of the ImmediateAbort panic strategy in a Cargo profile