memblock: Fix doc for memblock_phys_free

memblock_phys_free() is the counterpart to memblock_phys_alloc.
Change memblock_alloc_xx() with memblock_phys_alloc_xx() to keep
consistency.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: https://lore.kernel.org/r/20221216100304.688209-1-linmq006@gmail.com
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>