Fix typo in nnunet (Project-MONAI#1564)

Fixes #Project-MONAI/MONAI#7192

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Avoid including large-size files in the PR.
- [ ] Clean up long text outputs from code cells in the notebook.
- [ ] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [ ] 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>`

Signed-off-by: KumoLiu <yunl@nvidia.com>

nnunet/README.md

@@ -92,15 +92,15 @@ python -m monai.apps.nnunet nnUNetV2Runner run --input_config='./input.yaml' --t
 The supported `trainer_class_name` are:
 - nnUNetTrainer (default)
 - nnUNetTrainer_1epoch
-- nnUNetTrainer_5epoch
-- nnUNetTrainer_10epoch
-- nnUNetTrainer_20epoch
-- nnUNetTrainer_50epoch
-- nnUNetTrainer_100epoch
-- nnUNetTrainer_250epoch
-- nnUNetTrainer_2000epoch
-- nnUNetTrainer_4000epoch
-- nnUNetTrainer_8000epoch
+- nnUNetTrainer_5epochs
+- nnUNetTrainer_10epochs
+- nnUNetTrainer_20epochs
+- nnUNetTrainer_50epochs
+- nnUNetTrainer_100epochs
+- nnUNetTrainer_250epochs
+- nnUNetTrainer_2000epochs
+- nnUNetTrainer_4000epochs
+- nnUNetTrainer_8000epochs
 
 ### Run nnU-Net modules using ```nnUNetV2Runner```