[docs] A few tweaks to prefix_scan example.
[docs] A few tweaks to prefix_scan example.

Makes sure it matches the in-repo example, though notably the multidimensional
array changes from #513 have already been fixed.

Fixes #513

PiperOrigin-RevId: 423975870

google/xls,google/xls,google/xls,google/xls,google/xls,google/xls