docs: ext4: modify the group desc size to 64

Since the default ext4 group desc size is 64 now (assuming that the
64-bit feature is enbled). And the size mentioned in this doc is 64 too.
Change it to 64.

Signed-off-by: Wu Bo <[e-mail]>
Acked-by: Darrick J. Wong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

Documentation/filesystems/ext4/blockgroup.rst

···
 block is placed at the beginning of the first, second, and last block
 groups in a meta-block group. A meta-block group is a collection of
 block groups which can be described by a single block group descriptor
-block. Since the size of the block group descriptor structure is 32
-bytes, a meta-block group contains 32 block groups for filesystems with
-a 1KB block size, and 128 block groups for filesystems with a 4KB
+block. Since the size of the block group descriptor structure is 64
+bytes, a meta-block group contains 16 block groups for filesystems with
+a 1KB block size, and 64 block groups for filesystems with a 4KB
 blocksize. Filesystems can either be created using this new block group
 descriptor layout, or existing filesystems can be resized on-line, and
 the field s_first_meta_bg in the superblock will indicate the first