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 <bo.wu@vivo.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20230222013525.14748-1-bo.wu@vivo.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>