iotests: Fix typo in iotest 051

There is an typo in iotest 051, correct it.

Signed-off-by: Tao Xu <tao3.xu@intel.com>
Message-Id: <20210324084321.90952-1-tao3.xu@intel.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>