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GH-41829: [R] Update relative URLs in README to absolute paths to prevent CRAN check failures  (#41830)

### Rationale for this change

Relative URLs in README mean we fail CRAN checks

### What changes are included in this PR?

Update relative URLs to absolute URLs

### Are these changes tested?

Nope

### Are there any user-facing changes?

Nope
* GitHub Issue: #41829

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>