[net_processing] fix compact block related comment

Summary:
There are only version=1 compactblocks in Bitcoin ABC.
This was overlooked in rABC103cbb8e035745e9303350d57d85b32c1330cdf5

Test Plan: proof-reading

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14757