drivers/base/memory: Fix comments for phys_index_show()

drivers/base/memory: Fix comments for phys_index_show()

According to 'admin-guide/mm/memory-hotplug.rst', the memory block ID,
instead of the section index, is shown by '/sys/devices/system/memory/
memoryX/phys_index'.

Fix the comments to match with 'admin-guide/mm/memory-hotplug.rst'.
Besides, use the existing helper memory_block_id() to convert the section
index to the memory block index.

No functional change intended.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Link: https://lore.kernel.org/r/20230120055727.355483-2-gshan@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>