migration/postcopy: fix document of postcopy_send_discard_bm_ram()

migration/postcopy: fix document of postcopy_send_discard_bm_ram()

Commit 6b6712efccd3 ('ram: Split dirty bitmap by RAMBlock') changes the
parameter of postcopy_send_discard_bm_ram(), while left the document
part untouched.

This patch correct the document and fix two typo by hand.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190715020549.15018-1-richardw.yang@linux.intel.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>