documentation: vm: Advertise support for pte_special in riscv

Risc-V architecture has actually supported pte_special since its merge
upstream, simply add this info to the documentation.
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/features/vm/pte_special/arch-support.txt

@@ -23,7 +23,7 @@
     |    openrisc: | TODO |
     |      parisc: | TODO |
     |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: |  ok  |
     |       sparc: |  ok  |