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MINOR: [Docs] Update alignment to center for ADBC documentation (#43081)



### Rationale for this change

Update alignment to center in specification for ADBC

### What changes are included in this PR?

Update alignment to some content.

### Are these changes tested?

Yes. I have verified the changes by building the documentation.

### Are there any user-facing changes?

Yes. the updated documentation will be visible to users.

Authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>