Tutorial: Fix page count

It was incorrectly set to 421 while we actually have 471 images.

We need to update:

• tutorial/corpus/#data-partitioning counts
• `tutorial/corpus/#visualize-your-dataset screenshot
• tutorial/segmentation-ground-truth/#invite-collaborators tasks list screenshot
• tutorial/segmentation-ground-truth/#request-annotation-tasks screenshots
• tutorial/segmentation-ground-truth/#track-and-export-annotations-back-to-arkindex screenshots
• tutorial/transcription-ground-truth/#import-an-arkindex-dataset screenshots
• tutorial/transcription-ground-truth/#request-annotation-tasks screenshots

I might have missed some.