update error explanation in parameters fold in tutorials train_controlnet.ipynb (#1974)

Fixes # .

Description
The current documentation incorrectly states that "fold 0 is used for
training" in the prepare_maisi_controlnet_json_dataloader function. In
reality, the function uses the fold parameter to determine the
validation set based on matching fold indices. Specifically, if a
dataset item's fold value matches the fold value specified in
config_maisi_controlnet_train.json, it is assigned to the validation
set; otherwise, it is used for training.

Changes
Updated Documentation:
Revised the description of the fold parameter to accurately reflect its
functionality.
Clarified that in the current parameter settings, fold 0 is designated
for validation and fold 1 is designated for training.

---------

Signed-off-by: MaybeRichard <lisicheng168@gmail.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: MaybeRichard

generation/maisi/maisi_train_controlnet_tutorial.ipynb

@@ -189,7 +189,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 4,
+   "execution_count": null,
    "id": "fc32a7fe",
    "metadata": {},
    "outputs": [],
@@ -201,7 +201,9 @@
     "            \"image\": \"tr_image_001_emb.nii.gz\",  # relative path to the image embedding file\n",
     "            # relative path to the combined label (pseudo whole-body segmentation mask + ROI mask) file\n",
     "            \"label\": \"tr_label_001.nii.gz\",\n",
-    "            \"fold\": 0,  # fold index for cross validation, fold 0 is used for training\n",
+    "            \"fold\": 0,  # fold index for cross validation. If the dataset item's fold value is equal to the fold value\n",
+    "            # in config_maisi_controlnet_train.json, then it is used for validation. Otherwise, it is used for training.\n",
+    "            # In the current parameters, fold 0 is for validation.\n",
     "            \"dim\": sim_dim,  # the dimension of image\n",
     "            \"spacing\": [1.5, 1.5, 1.5],  # the spacing of image\n",
     "        },\n",