docs: fix typo "UNet_meatdata" → "UNet_metadata" (#2034)

### Summary
Fix a minor typo: "UNet_meatdata" → "UNet_metadata".

### Why
Correct spelling improves clarity and avoids confusion.

### Changes
- Updated spleen_segmentation_3d_visualization_basic.ipynb to use
correct key `UNet_metadata`.

### Validation
- Searched repository for `meatdata` to confirm no other occurrences.

### Issue
Fixes tutorials#2015

### Notes
- Documentation only, no code logic changes.

---------

Signed-off-by: minsu <minsu.kim08@gmail.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: minsuking

README.md

@@ -386,3 +386,4 @@ Example shows the use cases of using MONAI to evaluate the performance of a gene
 
 #### [VISTA2D](./vista_2d)
 This tutorial demonstrates how to train a cell segmentation model using the [MONAI](https://monai.io/) framework and the [Segment Anything Model (SAM)](https://github.com/facebookresearch/segment-anything) on the [Cellpose dataset](https://www.cellpose.org/).
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