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.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>