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MINOR: [Docs] Fix typo in docs for dense union data type (#35887)

## Rationale for this change / What changes are included in this PR?

Fixed typo in `Columnar.rst`. 

I think this typo may have crept in from an [older](https://wesm.github.io/arrow-site-test/format/Layout.html#example-layout-dense-union) version of the docs when validity bitmaps were allowed for Union types.

## Are these changes tested?

N/A

## Are there any user-facing changes?

Only in the docs.

Authored-by: Nick Byrne <byrnen8@tcd.ie>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>