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MINOR: [Format] Schema.fbs typo -- s/maybe/may be/ (#45023)

### Rationale for this change

To improve a comment.

### What changes are included in this PR?

Correction of a mispelling in a comment: s/maybe/may be/.

### Are these changes tested?

N/A

### Are there any user-facing changes?

No

Lead-authored-by: Alex Kesling <alex@kesling.co>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>