Add missing release note

Author: charlotte-hoblik

docs/release-notes/index.md

@@ -361,6 +361,7 @@ Machine Learning:
 * Mark token pruning for sparse vector as GA [#128854](https://github.com/elastic/elasticsearch/pull/128854)
 * Move to the Cohere V2 API for new inference endpoints [#129884](https://github.com/elastic/elasticsearch/pull/129884)
 * Semantic Text Chunking Indexing Pressure [#125517](https://github.com/elastic/elasticsearch/pull/125517)
+* Track memory used in the hierarchical results normalizer [#2831](https://github.com/elastic/ml-cpp/pull/2831)
 * Upgrade AWS v2 SDK to 2.30.38 [#124738](https://github.com/elastic/elasticsearch/pull/124738)
 * [Inference API] Propagate product use case http header to EIS [#124025](https://github.com/elastic/elasticsearch/pull/124025)
 * [ML] Add HuggingFace Chat Completion support to the Inference Plugin [#127254](https://github.com/elastic/elasticsearch/pull/127254)