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MINOR: [Go] Correct benchmark units from bits/s to bytes/s (#37479)



### Rationale for this change

We're trying to clean up the set of units used in Conbench, and the Go benchmarks stick out as `b/s` (bits per second), but looking at 

https://github.com/apache/arrow/blob/602083be4f9630eda3edaf3ac18a660ccce91468/ci/scripts/go_bench_adapt.py#L86

it looks like they're actually in `B/s` (bytes per second).

### What changes are included in this PR?

Just changing the units submitted to Conbench

### Are these changes tested?

No, but the unit will be validated by the Conbench API

### Are there any user-facing changes?

No

Authored-by: Edward Visel <1693477+alistaire47@users.noreply.github.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>