Fix stop key of TextGen. (#11109)

The key of stopping strings used in text-generation-webui api is
[`stopping_strings`](https://github.com/oobabooga/text-generation-webui/blob/main/api-examples/api-example.py#L51),
not `stop`.
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libs/langchain/langchain/llms/textgen.py

@@ -175,7 +175,7 @@ class TextGen(LLM):
             params = {"preset": self.preset}
 
         # then sets it as configured, or default to an empty list:
-        params["stop"] = self.stopping_strings or stop or []
+        params["stopping_strings"] = self.stopping_strings or stop or []
 
         return params