memremap.h: correct an error in a comment

It tried to send me off to memory_hotplug.h for an enum that is a few
lines above...

Link: https://lkml.kernel.org/r/dba0f5f01162d6fa16e4da2a9fede7f97080e92d.1707179960.git.john@groves.net


Signed-off-by: John Groves <john@groves.net>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>