Docs/mm: Fix a mistake for pfn in page_tables.rst

The documentation incorrectly calculate the pfn value as 0x3fffff,
which should be 0x3ffff instead. It is obtained by right-shifting
0xffffc000 by 14 bits.

This patch corrects the value to prevent any potential confusion
for developers referencing this document.

Signed-off-by: Pengyu Zhang <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Zenghui Yu <[e-mail]>
Reviewed-by: Mike Rapoport (Microsoft) <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/mm/page_tables.rst

···
 With a page granularity of 4KB and a address range of 32 bits, pfn 0 is at
 address 0x00000000, pfn 1 is at address 0x00001000, pfn 2 is at 0x00002000
 and so on until we reach pfn 0xfffff at 0xfffff000. With 16KB pages pfs are
-at 0x00004000, 0x00008000 ... 0xffffc000 and pfn goes from 0 to 0x3fffff.
+at 0x00004000, 0x00008000 ... 0xffffc000 and pfn goes from 0 to 0x3ffff.
 
 As you can see, with 4KB pages the page base address uses bits 12-31 of the
 address, and this is why `PAGE_SHIFT` in this case is defined as 12 and