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>