docs: f2fs: fix html doc error

There is a problem with the html converted from the previous
commit 6047de54 ("f2fs: add barrier mount option")
code submission. Probably something like this:

barrier If this				option is set, cache_flush commands are allowed to be

Let's fix it.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>