docs: mention moov-io/paygate compared to moov-io/ach
docs: mention moov-io/paygate compared to moov-io/ach

Fixes: https://github.com/moov-io/ach/issues/507

wadearnold/ach,wadearnold/ach,moov-io/ach,moov-io/ach,moov-io/ach,moov-io/ach