Update whats_new.md

Author: cmacdonald

doc/whats_new.md

@@ -6,7 +6,7 @@ Terrier 5.10 - 22/8/2024
 
 Significant update, adding functionality useful for PyTerrier use cases, and some other small minor improvements.
 
-## Retrieval
+## Retrieval
 
  * Include RM1 and RM3 query expansion models, originally authored by @ntonellotto, and previously included in the separate [terrier-prf package](https://github.com/terrierteam/terrier-prf). This integration aims to simplify the usage in PyTerrier [#250](https://github.com/terrier-org/terrier-core/pull/250), [#253](https://github.com/terrier-org/terrier-core/pull/253). Thanks to @mam10eks for RM3 improvements.
  * Include previously unreleased doc-vectors module authored by @cmacdonald [#249](https://github.com/terrier-org/terrier-core/pull/249), which allows calculation of additional weighting model features using direct index rather than the inverted index access provided by Fat. For more information, see the relevant [README](https://github.com/terrier-org/terrier-core/blob/5.x/modules/docvectors/README.md). This functionality can be accessed through PyTerrier for the purposes of learning-to-rank pipelines by using `FeaturesBatchRetrieve(..., method='dv')`.