Update mongodb_atlas.py docstrings (#8033)

Hi all, I just added the "index_name" parameter to the docstrings for
mongodb_atlas.py (it is missing in the [public doc
page](https://api.python.langchain.com/en/latest/vectorstores/langchain.vectorstores.mongodb_atlas.MongoDBAtlasVectorSearch.html#langchain-vectorstores-mongodb-atlas-mongodbatlasvectorsearch).

Thanks

langchain/vectorstores/mongodb_atlas.py

@@ -69,6 +69,7 @@ class MongoDBAtlasVectorSearch(VectorStore):
                 document.
             embedding_key: MongoDB field that will contain the embedding for
                 each document.
+            index_name: Name of the Atlas Search index.
         """
         self._collection = collection
         self._embedding = embedding