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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/ext4/super.rst

@@ -456,15 +456,15 @@ The ext4 superblock is laid out as follows in
    * - 0x277
      - __u8
      - s_lastcheck_hi
-     - Upper 8 bits of the s_lastcheck_hi field.
+     - Upper 8 bits of the s_lastcheck field.
    * - 0x278
      - __u8
      - s_first_error_time_hi
-     - Upper 8 bits of the s_first_error_time_hi field.
+     - Upper 8 bits of the s_first_error_time field.
    * - 0x279
      - __u8
      - s_last_error_time_hi
-     - Upper 8 bits of the s_last_error_time_hi field.
+     - Upper 8 bits of the s_last_error_time field.
    * - 0x27A
      - __u8
      - s_pad[2]