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MINOR: [C++] Fix build issue where wrong variable checked for re2 prefix dir (#34735)

### Rationale for this change

CMake variables are case sensitive.

When providing a manual prefix dir for re2 (`re2_ROOT`), the wrong variable (`RE2_ROOT`) would be checked, causing build issues.

### What changes are included in this PR?

typo fix

### Are these changes tested?

Built on my end by providing `-Dre2_ROOT=...`

### Are there any user-facing changes?

Possibly if a user decided to set `RE2_ROOT` to something different. Unlikely, though.

Authored-by: Mike Lui <mikelui@meta.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>