Doc: treewide : Fix typos in DocBook/filesystem.xml

This patch fix spelling typos found in DocBook/filesystem.xml.
It is because the file was generated from comments in code,
I have to fix the comments in codes, instead of xml file.

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