obj/riscv: fix link to risc-v dwarf register numbers

The repository name and structure in the RISC-V GitHub org has been
modified, rendering the existing link invalid. This updates to point at
the new location of the RISC-V DWARF specification.

Change occured in https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/208

Change-Id: I8ca4c390bee2d7ce20418cdd00e4945a426cf5f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/363355


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Than McIntosh <thanm@google.com>