migration: fix a typo

'postocpy' should be 'postcopy'.

CC: qemu-trivial@nongnu.org
Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20190525062832.18009-1-liq3ea@163.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>