KVM: fix typo in documentation

The documentation mentions a non-existing capability KVM_CAP_USER_MEM.s
The right name is KVM_CAP_USER_MEMORY.
Signed-off-by: NDennis Restle <derestle@htwg-konstanz.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -1079,7 +1079,7 @@ yet and must be cleared on entry.
 
 4.35 KVM_SET_USER_MEMORY_REGION
 
-Capability: KVM_CAP_USER_MEM
+Capability: KVM_CAP_USER_MEMORY
 Architectures: all
 Type: vm ioctl
 Parameters: struct kvm_userspace_memory_region (in)