jbd2: fix kernel-doc notation warning

Fix kernel-doc warning by inserting a beginning '*' character
for the kernel-doc line.

../include/linux/jbd2.h:461: warning: bad line:         journal. These are dirty buffers and revoke descriptor blocks.

Link: https://lore.kernel.org/r/53e3ce27-ceae-560d-0fd4-f95728a33e12@infradead.org
Cc: stable@kernel.org
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>