Documentation: riscv: Fix typo MIMPLID -> MIMPID

Documentation: riscv: Fix typo MIMPLID -> MIMPID

The macro that is really defined is RISCV_HWPROBE_KEY_MIMPID, not
RISCV_HWPROBE_KEY_MIMPLID (difference is the 'L').

Also, the riscv privileged specification names the register "mimpid", not
"mimplid".

Correct these typos.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20240925142532.31808-1-namcao@linutronix.de
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>