VDS Combiner code conflicts with documentation

[MattWellie]

hail/hail/python/hail/vds/combiner/variant_dataset_combiner.py

Line 548 in bf3c9da

for start in range(0, len(vcfs), step):

In the documentation:

branch_factor – The number of Variant Datasets to combine at once.
gvcf_batch_size – The number of GVCFs to combine into a Variant Dataset at once.

In this line the number of gVCFs being merged into each vds is step, and the value of step is self.branch_factor, when it should be gvcf_batch_size.

First spotted by @patrick-schultz (https://hail.zulipchat.com/#narrow/channel/123010-Hail-Query-0.2E2-support/topic/gVCF.20Combiner.20Save.20Path.2FPlan/near/492731854)