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MINOR: [R] Fix headers (#44960)

### Rationale for this change

The headers in NEWS.md didn't reflect the versions we actually released. We didn't release an 18.0.0 version of the package and waited for Arrow C++ 18.1.0 to do the first R package release in the 18.x.x series.

### What changes are included in this PR?

Update to NEWS.md.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>