Restrict Auto3DSeg fold input based on datalist (#7778)

Fixes #7777.

### Description

Lower the maximum `num_fold` allowed for user inputs in Auto3DSeg
AutoRunner

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [x] In-line docstrings updated.

Signed-off-by: Mingxin Zheng <mingxinz@nvidia.com>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

monai/apps/auto3dseg/auto_runner.py

@@ -499,8 +499,8 @@ def set_num_fold(self, num_fold: int = 5) -> AutoRunner:
 
         if num_fold <= 0:
             raise ValueError(f"num_fold is expected to be an integer greater than zero. Now it gets {num_fold}")
-        if num_fold > self.max_fold + 1:
-            # Auto3DSeg allows no validation set, so the maximum fold number is max_fold + 1
+        if num_fold > self.max_fold:
+            # Auto3DSeg must contain validation set, so the maximum fold number is max_fold.
             raise ValueError(
                 f"num_fold is greater than the maximum fold number {self.max_fold} in {self.datalist_filename}."
             )