DOCS: Fixed import of langchain instead of langchain_nvidia_ai_endpoints for ChatNVIDIA (#27734)
* **PR title**: "docs: Replaced langchain import with
langchain-nvidia-ai-endpoints in NVIDIA Endpoints Tab"
* **PR message**:
+ **Description:** Replaced the import of `langchain` with
`langchain-nvidia-ai-endpoints` in the NVIDIA Endpoints Tab to resolve
an error caused by the documentation attempting to import the generic
`langchain` module despite the targeted import.
+ **Issue:**
+ **Dependencies:** No additional dependencies introduced; simply
updated the existing import to a more specific module.
+ **Twitter handle:** https://x.com/nawaz0x1
* **Add tests and docs**:
+ **Applicability:** Not applicable in this case, as the change is a fix
to an existing integration rather than the addition of a new one.
+ **Rationale:** No new functionality or integrations are introduced,
only a corrective import change.
* **Lint and test**:
+ **Status:** Completed
+ **Outcome:**
- `make format`: **Passed**
- `make lint`: **Passed**
- `make test`: **Passed**
