docs/mm: Physical Memory: fix a reference to a file that doesn't exist

docs/mm: Physical Memory: fix a reference to a file that doesn't exist

kbuild reports:

>> Warning: Documentation/mm/physical_memory.rst references a file that doesn't exist: Documentation/admin-guide/mm/memory_hotplug.rst

Fix the filename to be 'Documentation/admin-guide/mm/memory-hotplug.rst'.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202302231311.567PAoS2-lkp@intel.com/
Fixes: 353c7dd636ed ("docs/mm: Physical Memory: remove useless markup")
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230224100306.2287696-1-rppt@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>