Update OutputType in image_segmenter.md
The line 
```
segmentation_options = processor.SegmentationOptions(
    output_type=processor.SegmentationOptions.OutputType.CATEGORY_MASK) 
 ```
is changed to 
```
segmentation_options = processor.SegmentationOptions(
    output_type=processor.SegmentationOptions.output_type.CATEGORY_MASK) 
```
Please check this working [gist](https://colab.research.google.com/gist/pjpratik/2e15c0e8eeb262e7ce396ac4cbddb4bd/60836.ipynb).

Fixes #60836 