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MINOR: [Format] Schema.fbs grammar -- replace i.e. with e.g. (#45026)

### Rationale for this change

To improve a comment.

### What changes are included in this PR?

Correction of grammar in a comment: use e.g. in a parenthetical instead of i.e. 

For context: https://www.grammarly.com/blog/acronyms-abbreviations/i-e-vs-e-g/ .

### Are these changes tested?

N/A

### Are there any user-facing changes?

No

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