f2fs: fix to avoid adding tab before doc section

Otherwise whole section after tab will be invisible in compiled
html format document.

Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Fixes: 89272ca1 ("docs: filesystems: convert f2fs.txt to ReST")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>