docs: Fix formatting of literal sections in fanotify docs

Stephen Rothwell reported the following warning was introduced by commit
c0baf9ac ("docs: Document the FAN_FS_ERROR event").

Documentation/admin-guide/filesystem-monitoring.rst:60: WARNING:
 Definition list ends without a blank line; unexpected unindent.

Link: https://lore.kernel.org/r/87y26camhe.fsf@collabora.com


Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>