f2fs: fix wrong description of compress feature in f2fs.rst

f2fs will try compressing data in cluster only when "all logical
blocks in cluster contain valid data" rather than "all logical
blocks in file are valid".

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>