nicira-ext: Update comment.

The ARP headers have been acceptable as NXAST_REG_MOVE destinations since
commit f6c8a6b163 (Add software switch support for modifying ARP headers
in OpenFlow.)

Reported-by: Anupam Chanda <achanda@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>