dirstate-v2: Use "byte sequence" in docs
The patch originally sent as https://phab.mercurial-scm.org/D11546
used "byte string" but that was changed during review to avoid suggesting
Unicode or character encodings.
However "byte range" sounds to be like a range of *indices* within a byte
This changes to "byte sequence". Python docs use "sequence" a lot when
discussing the bytes type: https://docs.python.org/3/library/stdtypes.html
Differential Revision: https://phab.mercurial-scm.org/D11623