Documentation: filesystem: Fix typo in fs/eventfd.c

This patch fix typos found in Documentation/filesystems.xml,
DocBook/filesystems/API-eventfd-signal.html, and
DocBook/filesystems.aux.xml

These files are generated from comments within the source,
so I had to fix typos in fs/eventfd.c

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>