[RISCV] Fix the link to the XAndesPerf specification. NFC (#138804)

We need to use 2 underscores after the URL like the other specification
links.

Author: topperc

llvm/docs/RISCVUsage.rst

@@ -506,7 +506,7 @@ The current vendor extensions supported are:
   LLVM implements `version 0.1 of the Rivos Vector Register Zips extension specification <https://github.com/rivosinc/rivos-custom-extensions>`__.
 
 ``XAndesPerf``
-  LLVM implements `version 5.0.0 of the Andes Performance Extension specification <https://github.com/andestech/andes-v5-isa/releases/download/ast-v5_4_0-release/AndeStar_V5_ISA_Spec_UM165-v1.5.08-20250317.pdf>` by Andes Technology. All instructions are prefixed with `nds.` as described in the specification.
+  LLVM implements `version 5.0.0 of the Andes Performance Extension specification <https://github.com/andestech/andes-v5-isa/releases/download/ast-v5_4_0-release/AndeStar_V5_ISA_Spec_UM165-v1.5.08-20250317.pdf>`__ by Andes Technology. All instructions are prefixed with `nds.` as described in the specification.
 
 Experimental C Intrinsics
 =========================