arrow

MINOR: [R] Fix broken link in README.md (#38657)


### Rationale for this change

There is a broken link in README.md that refers to an old URL in the Arrow documentation. This causes the CRAN incoming check to fail.

### What changes are included in this PR?

The link was updated.

### Are these changes tested?

Not needed (docs only)

### Are there any user-facing changes?

No

Lead-authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>