Update custom_diffusion.mdx to credit the author (#3163)

* Update custom_diffusion.mdx

* fix: unnecessary list comprehension.

docs/source/en/training/custom_diffusion.mdx

@@ -15,6 +15,8 @@ specific language governing permissions and limitations under the License.
 [Custom Diffusion](https://arxiv.org/abs/2212.04488) is a method to customize text-to-image models like Stable Diffusion given just a few (4~5) images of a subject.
 The `train_custom_diffusion.py` script shows how to implement the training procedure and adapt it for stable diffusion.
 
+This training example was contributed by [Nupur Kumari](https://nupurkmr9.github.io/) (one of the authors of Custom Diffusion). 
+
 ## Running locally with PyTorch
 
 ### Installing the dependencies