doc: virtual: Fix typos in virtio-spec.txt

Correct spelling typo in documentation/virtual/virtio-spec.txt

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>