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

kbuild reports:

>> Warning: Documentation/mm/hugetlbfs_reserv.rst references a file that doesn't exist: Documentation/mm/hugetlbpage.rst
>> Warning: Documentation/translations/zh_CN/mm/hugetlbfs_reserv.rst references a file that doesn't exist: Documentation/mm/hugetlbpage.rst

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

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202302231854.sKlCmx9K-lkp@intel.com/


Fixes: ee865889 ("docs/mm: remove useless markup")
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230224100306.2287696-2-rppt@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>