README for MSMARCO v2 doesn't align with corpus augmentation script

[mbarretta]

The changes introduced in https://github.com/castorini/docTTTTTquery/pull/66 changed, among other things, the arguments for augment_corpus.py. The description of that PR references that the PR for the README would occur separately, but I don't see one pending.

Consequently, the instructions for MS MARCO V2 Passage Expansion and MS MARCO V2 (Segmented) Document Expansion are currently incorrect.

Please update the README so folks like me can replicate your results!