Change the feature used as an example of stabilizing lib features (#1143)

Author: lf-

src/stability.md

@@ -85,7 +85,10 @@ To stabilize a feature, follow these steps:
    - Add the appropriate labels: `@rustbot modify labels: +T-libs`.
    - Link to the tracking issue and say "Closes #XXXXX".
 
-You can see an example of stabilizing a feature at [#75132](https://github.com/rust-lang/rust/pull/75132).
+You can see an example of stabilizing a feature with
+[tracking issue #81656 with FCP](https://github.com/rust-lang/rust/issues/81656)
+and the associated
+[implementation PR #84642](https://github.com/rust-lang/rust/pull/84642).
 
 ## allow_internal_unstable