update monai nvflare example links (#2062)

Fixes # .

### Description
Update link for NVFlare based Federated Learning examples to link to the
latest version in the NVFlare repo.

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Avoid including large-size files in the PR.
- [x] Clean up long text outputs from code cells in the notebook.
- [x] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [x] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [ ] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated documentation references for NVFlare MONAI FL integration
examples to point to the latest stable version.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Holger Roth <hroth@nvidia.com>

Author: holgerroth

federated_learning/nvflare/README.md

@@ -1,4 +1,4 @@
 # Federated learning with NVIDIA FLARE
 [NVFlare](https://pypi.org/project/nvflare/) provides examples on how to train federated learning models using MONAI's [FL components](https://docs.monai.io/en/latest/fl.html).
 
-## Please visit [here](https://github.com/NVIDIA/NVFlare/tree/dev/integration/monai/examples).
+## Please visit [here](https://github.com/NVIDIA/NVFlare/tree/main/integration/monai).