Fix minor bug in opensearch vector store add_texts function (#1878)

In the langchain.vectorstores.opensearch_vector_search.py, in the
add_texts function, around line 247, we have the following code

```python
embeddings = [
     self.embedding_function.embed_documents(list(text))[0] for text in texts
]
```

the goal of the `list(text)` part I believe is to pass a list to the
embed_documents list instead of a a str. However, `list(text)` is a
subtle bug

`list(text)` would convert the string text into an array, where each
element of the array is a character of the string

<img width="937" alt="Screenshot 2023-03-22 at 1 27 18 PM"
src="https://user-images.githubusercontent.com/88190553/226836470-384665a1-2f13-46bc-acfc-9a37417cd918.png">

The correct way should be to change the code to 

```python
embeddings = [
      self.embedding_function.embed_documents([text])[0] for text in texts
]
```
Which wraps the string inside a list.

langchain/vectorstores/opensearch_vector_search.py

@@ -243,7 +243,7 @@ class OpenSearchVectorSearch(VectorStore):
             List of ids from adding the texts into the vectorstore.
         """
         embeddings = [
-            self.embedding_function.embed_documents(list(text))[0] for text in texts
+            self.embedding_function.embed_documents([text])[0] for text in texts
         ]
         _validate_embeddings_and_bulk_size(len(embeddings), bulk_size)
         return _bulk_ingest_embeddings(