[llvm] [RISCV] Update SiFive VCIX documentation link. NFC (PR #96986)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM.
(I added backticks to `__riscv_` in the description or it will be italic __riscv_)

https://github.com/llvm/llvm-project/pull/96986