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>

Documentation/filesystems/f2fs.rst

@@ -158,7 +158,7 @@ nobarrier		 This option can be used if underlying storage guarantees
 			 If this option is set, no cache_flush commands are issued
 			 but f2fs still guarantees the write ordering of all the
 			 data writes.
-barrier		 If this option is set, cache_flush commands are allowed to be
+barrier			 If this option is set, cache_flush commands are allowed to be
 			 issued.
 fastboot		 This option is used when a system wants to reduce mount
 			 time as much as possible, even though normal performance