KVM: Fix typos in Documentation/kvm/mmu.txt

Guest memory (gpa) is part of the user address space of the process that is

using kvm.  Userspace defines the translation between guest addresses and user

addresses (gpa->hva); note that two gpas may alias to the same gva, but not

addresses (gpa->hva); note that two gpas may alias to the same hva, but not

vice versa.

These gvas may be backed using any method available to the host: anonymous

These hvas may be backed using any method available to the host: anonymous

memory, file backed memory, and device memory.  Memory might be paged by the

host at any time.