RISC-V: Fix a comment typo in riscv_of_parent_hartid()

This fixes a typo in a comment that is both obvious and went unnoticed.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Fixes: a9b20260 ("RISC-V: Improve /proc/cpuinfo output for ISA extensions")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>