Documentation: ext4: correct the document about superblock

Documentation: ext4: correct the document about superblock

The description of s_lastcheck_hi, s_first_error_time_hi, and
s_last_error_time_hi fields refer to themselves, while these means
referring to upper 8 bits (byte) of corresponding fields (s_lastcheck,
s_first_error_time, and s_last_error_time). Correct the mistake.

Signed-off-by: JunChao Sun <sunjunchao2870@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20220815125233.2040-1-sunjunchao2870@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>