docs: f2fs: fixed spelling mistakes in documentation

found/fixed the following typos

- deivces -> devices
- substracting -> subtracting

in `Documentation/ABI/testing/sysfs-fs-f2fs`

Signed-off-by: Soham Metha <sohammetha01@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>