cmd/go: update link to RISC-V profile doc

cmd/go: update link to RISC-V profile doc

The URL has changed.

Change-Id: I295020c59aaf64a89c4307fca527163095148a67
Reviewed-on: https://go-review.googlesource.com/c/go/+/591897
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: M Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>