docs: Update IBM WatsonxRerank documentation (#29138)

Thank you for contributing to LangChain!

Update presented model in `WatsonxRerank` documentation.

- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

docs/docs/integrations/retrievers/ibm_watsonx_ranker.ipynb

@@ -194,7 +194,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 6,
+   "execution_count": null,
    "id": "029dc5e7",
    "metadata": {},
    "outputs": [],
@@ -291,7 +291,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 4,
+   "execution_count": null,
    "id": "a6705dda",
    "metadata": {},
    "outputs": [],
@@ -299,7 +299,7 @@
     "from langchain_ibm import WatsonxRerank\n",
     "\n",
     "wx_rerank = WatsonxRerank(\n",
-    "    model_id=\"ibm/slate-125m-english-rtrvr\",\n",
+    "    model_id=\"cross-encoder/ms-marco-minilm-l-12-v2\",\n",
     "    url=\"https://us-south.ml.cloud.ibm.com\",\n",
     "    project_id=\"PASTE YOUR PROJECT_ID HERE\",\n",
     ")"