mm, memory_hotplug: update a comment in unregister_memory()

The remove_memory_block() function was renamed to in commit
cc292b0b ("drivers/base/memory.c: rename remove_memory_block() to
remove_memory_section()").

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>