docs(langchain): correct IBM provider link in chat_models docstring (#33897)

**PR title**

```
docs(langchain): correct IBM provider link in chat_models docstring
```

**PR message**

**Description**
Fix broken link in the `chat_models` docstring. The **ibm** bullet
incorrectly linked to the DeepSeek provider page; update it to the
canonical IBM provider docs.

This only affects generated API reference content on
`reference.langchain.com`. No runtime behavior changes.

**Issue**
N/A (documentation-only).

**Dependencies**
None.

**Testing & quality**

* Ran `make format`, `make lint`, and `make test` in the package (no
code changes expected to affect tests).

Author: artreimus

libs/langchain/langchain_classic/chat_models/base.py

@@ -123,7 +123,7 @@ def init_chat_model(
             - `ollama`                  -> [`langchain-ollama`](https://docs.langchain.com/oss/python/integrations/providers/ollama)
             - `google_anthropic_vertex` -> [`langchain-google-vertexai`](https://docs.langchain.com/oss/python/integrations/providers/google)
             - `deepseek`                -> [`langchain-deepseek`](https://docs.langchain.com/oss/python/integrations/providers/deepseek)
-            - `ibm`                     -> [`langchain-ibm`](https://docs.langchain.com/oss/python/integrations/providers/deepseek)
+            - `ibm`                     -> [`langchain-ibm`](https://docs.langchain.com/oss/python/integrations/providers/ibm)
             - `nvidia`                  -> [`langchain-nvidia-ai-endpoints`](https://docs.langchain.com/oss/python/integrations/providers/nvidia)
             - `xai`                     -> [`langchain-xai`](https://docs.langchain.com/oss/python/integrations/providers/xai)
             - `perplexity`              -> [`langchain-perplexity`](https://docs.langchain.com/oss/python/integrations/providers/perplexity)