Documentation: RISC-V: hwprobe: Fix a formatting error

I'm not sure what I was trying to do with the ':'s, but they're just
rendered to HTML which looks odd.   This makes "fence.i" look like
"mvendorid" and such, which is seems reasonable to me.

Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20230710193329.2742-1-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: palmer-dabbelt

Documentation/riscv/hwprobe.rst

@@ -49,7 +49,7 @@ The following keys are defined:
     privileged ISA, with the following known exceptions (more exceptions may be
     added, but only if it can be demonstrated that the user ABI is not broken):
 
-    * The :fence.i: instruction cannot be directly executed by userspace
+    * The ``fence.i`` instruction cannot be directly executed by userspace
       programs (it may still be executed in userspace via a
       kernel-controlled mechanism such as the vDSO).