misc: update mke2fs's man page regarding the default inode size

misc: update mke2fs's man page regarding the default inode size

Since a23b50cd ("mke2fs: warn about missing y2038 support when
formatting fresh ext4 fs"), the default inode size is 256 bytes
for all filesystems, including small and floppy, except for the
Hurd since it currently only supports 128-byte inodes.

Signed-off-by: Pascal Hambourg <pascal@plouf.fr.eu.org>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/E1rx4t4-00073d-1e@zenith
Signed-off-by: Theodore Ts'o <tytso@mit.edu>