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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