x86/xen: Fix typo in xen_pagetable_p2m_free()

Fix typo in xen_pagetable_p2m_free(): s/Fortunatly/Fortunately

Signed-off-by: Hui Su <sh_def@163.com>
Link: https://lore.kernel.org/r/20200927172836.GA7423@rlk

[boris: reword commit message slightly]
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>