userfaultfd/selftests: fix typo in comment

Delete the redundant word 'in'.

Link: https://lkml.kernel.org/r/20220610071244.59679-1-wangxiang@cdjrlc.com


Signed-off-by: Xiang wangx <wangxiang@cdjrlc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>