selftests: futex: Fix 'the the' typo in comment

Replace 'the the' with 'the' in the comment.

Signed-off-by: Slark Xiao <slark_xiao@163.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20220725015903.5449-1-slark_xiao@163.com