riscv/mm: Fix the comment for swap pte format

Swap type takes bits 7-11 and swap offset should start from bit 12.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20230921141652.2657054-1-xiao.w.wang@intel.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>