[RISCV][Docs] Correct links to Xmipscmov and Xmipslsp specifications. NFC

Need to add 2 underscores after the URL.

Author: topperc

llvm/docs/RISCVUsage.rst

@@ -494,10 +494,10 @@ The current vendor extensions supported are:
   LLVM implements `version 0.2 of the Qualcomm uC Sync Delay extension specification <https://github.com/quic/riscv-unified-db/releases/latest>`__ by Qualcomm.  All instructions are prefixed with `qc.` as described in the specification. These instructions are only available for riscv32.
 
 ``Xmipscmov``
-  LLVM implements conditional move for the `p8700 processor <https://mips.com/products/hardware/p8700/>` by MIPS.
+  LLVM implements conditional move for the `p8700 processor <https://mips.com/products/hardware/p8700/>`__ by MIPS.
 
 ``Xmipslsp``
-  LLVM implements load/store pair instructions for the `p8700 processor <https://mips.com/products/hardware/p8700/>` by MIPS.
+  LLVM implements load/store pair instructions for the `p8700 processor <https://mips.com/products/hardware/p8700/>`__ by MIPS.
 
 ``experimental-XRivosVisni``
   LLVM implements `version 0.1 of the Rivos Vector Integer Small New Instructions extension specification <https://github.com/rivosinc/rivos-custom-extensions>`__.