drm/ttm: Fix ttm_backup kerneldoc

The docs were not properly updated from an earlier version of the code.

Fixes: e7b5d23e ("drm/ttm: Provide a shmem backup implementation")
Cc: Christian König <christian.koenig@amd.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250502130101.3185-1-thomas.hellstrom@linux.intel.com