docs: ABI: Fix the spelling oustanding to outstanding in the file sysfs-fs-xfs

s/oustanding/outstanding/

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>