bitset: __BIT_FFS_AT(): Fix herald comment, take 2

Remove the reference to the nonexistent 'end' parameter. While here,
rephrase a bit.

I did the initial comment fix (commit "bitset: Fix __BIT_FFS_AT()'s
herald comment", f3ab0d86e8070c73) as part of an experiment introducing
macros to operate on ranges of bits in a bitset and subject to
a predicate (a generalization of some code used in some pending
modifications of the ULE scheduler), which was finally ditched as being
too verbose and impractical to use. I however then forgot to remove the
reference to 'end'.

No functional change.

Noted by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
MFC with: f3ab0d86e807
Sponsored by: The FreeBSD Foundation