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MINOR: [Docs] Small grammar fix in Intro.rst (#45109)

Small grammar fix in the intro docs (missing `the`).

### Rationale for this change

Improving docs.

### What changes are included in this PR?

Small grammar fix in `docs/source/format/Intro.rst`

### Are these changes tested?

No, as these are only docs changes.

### Are there any user-facing changes?

No.

Authored-by: Tim Grein <grein.tim97@gmail.com>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>