Fix benchmark batch verification (tari-project#33)

The benchmark for batch verification always runs against a single proof due to a typo. This PR fixes the error.

benches/triptych.rs

@@ -189,7 +189,7 @@ fn verify_batch_proof(c: &mut Criterion) {
  );
  group.bench_function(&label, |b| {
  // Generate data
- let (witnesses, statements, transcripts) = generate_data(&params, 1, &mut rng);
+ let (witnesses, statements, transcripts) = generate_data(&params, batch, &mut rng);
 
  // Generate the proofs
  let proofs = izip!(witnesses.iter(), statements.iter(), transcripts.clone().iter_mut())