Clean todo comment in constant-riscv64.h about PCRelativeJumpRange.

Change-Id: I9067134e96e4801fbd1f976d0e5d033085d5f133
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2817975Reviewed-by: Brice Dobry <brice.dobry@futurewei.com>
Commit-Queue: Yahan Lu <yahan@iscas.ac.cn>
Cr-Commit-Position: refs/heads/master@{#73925}