ext4: fix Kconfig documentation for CONFIG_EXT4_DEBUG

Fox the Kconfig documentation for CONFIG_EXT4_DEBUG to match the
change made by commit a0b30c12: ext4: use module parameters instead
of debugfs for mballoc_debug

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org