mke2fs.8.in: clarify the sign of a block-size constraint.

mke2fs.8.in: clarify the sign of a block-size constraint.

This bit of the mke2fs manpage is slightly confusing:

-b block-size
    Specify the size of blocks in bytes. <snip>
    If block-size is negative, then mke2fs will use heuristics
    to determine the appropriate block size, with the constraint
    that the block size will be at least block-size bytes.

because it sounds like the block size will be at least a negative
number.  Clarify just what the negative sign means.

Reported-by: Chris Frost <chris@frostnet.net>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>