doc: fixed a typo in ext4 documentation

The unit of file system size should be TiB, not PiB
Signed-off-by: lianzhi chang <changlianzhi@uniontech.com>
Link: https://lore.kernel.org/r/20220310014415.29937-1-changlianzhi@uniontech.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

Documentation/filesystems/ext4/blocks.rst

@@ -39,7 +39,7 @@ For 32-bit filesystems, limits are as follows:
      - 4TiB
      - 8TiB
      - 16TiB
-     - 256PiB
+     - 256TiB
    * - Blocks Per Block Group
      - 8,192
      - 16,384