documentation: Fix riscv cmodx example

ON/OFF in the keys was swapped between the first and second argument of
the prctl. The prctl key is always PR_RISCV_SET_ICACHE_FLUSH_CTX, and
the second argument can be PR_RISCV_CTX_SW_FENCEI_ON or
PR_RISCV_CTX_SW_FENCEI_OFF.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Fixes: 6a08e470 ("documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl")
Link: https://lore.kernel.org/r/20240628-fix_cmodx_example-v1-1-e6c6523bc163@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>