community[patch]: Fix KeyError 'embedding' (MongoDBAtlasVectorSearch) (#17178)

- **Description:**
Embedding field name was hard-coded named "embedding".
So I suggest that change `res["embedding"]` into
`res[self._embedding_key]`.
  - **Issue:** #17177,
- **Twitter handle:**
[@bagcheoljun17](https://twitter.com/bagcheoljun17)

libs/community/langchain_community/vectorstores/mongodb_atlas.py

@@ -209,7 +209,7 @@ class MongoDBAtlasVectorSearch(VectorStore):
         for res in cursor:
             text = res.pop(self._text_key)
             score = res.pop("score")
-            del res["embedding"]
+            del res[self._embedding_key]
             docs.append((Document(page_content=text, metadata=res), score))
         return docs