Update default engine from nmslib to faiss (#8620)

* Update default engine from nmslib to faiss

Starting 2.18, default knn engine is faiss. Updating the documentation
accordingly

Signed-off-by: Vijayan Balasubramanian <balasvij@amazon.com>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

---------

Signed-off-by: Vijayan Balasubramanian <balasvij@amazon.com>
Signed-off-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>
Co-authored-by: Naarcha-AWS <97990722+Naarcha-AWS@users.noreply.github.com>

_search-plugins/knn/knn-index.md

@@ -92,7 +92,7 @@ Mapping parameter | Required | Default | Updatable | Description
 :--- | :--- | :--- | :--- | :---
 `name` | true | n/a | false | The identifier for the nearest neighbor method.
 `space_type` | false | l2 | false | The vector space used to calculate the distance between vectors. Note: This value can also be specified at the top level of the mapping.
-`engine` | false | nmslib | false | The approximate k-NN library to use for indexing and search. The available libraries are faiss, nmslib, and Lucene.
+`engine` | false | Faiss  | false | The approximate k-NN library to use for indexing and search. The available libraries are Faiss, nmslib, and Lucene.
 `parameters` | false | null | false | The parameters used for the nearest neighbor method.
 
 ### Supported nmslib methods