docs: List kvm as a supported accelerator on RISC-V

Since commit fbf43c7dbf ("target/riscv: enable riscv kvm accel"),
KVM accelerator is supported on RISC-V. Let's document it.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220719082635.3741878-1-bin.meng@windriver.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/about/build-platforms.rst

@@ -46,7 +46,7 @@ Those hosts are officially supported, with various accelerators:
    * - PPC
      - kvm, tcg
    * - RISC-V
-     - tcg
+     - kvm, tcg
    * - s390x
      - kvm, tcg
    * - SPARC