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GH-37019: [R] Documentation for read_parquet() et al needs updating (#37020)

### Rationale for this change

Docs were out of data with code after previous changes to returned object type

### What changes are included in this PR?

Update docs to reflect correct return type

### Are these changes tested?

No

### Are there any user-facing changes?

No
* Closes: #37019

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