selftests: typo correction for memory-hotplug test

selftests: typo correction for memory-hotplug test

BugLink: http://bugs.launchpad.net/bugs/1710868
Typo fixed for hotpluggable_offline_memory() in memory-hotplug test.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
(cherry picked from commit 593f927851a58fdca559b173c807a0b78c9f6e49)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>