f2fs: fix description about discard_granularity node

Let's fix the inconsistency in the text description.
Default discard granularity is 16. For small devices,
default value is 1.

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