tools:testing/selftests: fix typo in futex/README

Correct typo in tools/testing/selftests/futex/README.

Signed-off-by: Wei Jiangang <weijg.fnst@cn.fujitsu.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>