message: Fix obsoleted link to tech spec
diff: @@ -6,7 +6,7 @@ Specification
  There is a draft specification currently located at
  -https://github.com/BitcoinUnlimited/BUIP/blob/master/BUIP-UAHF/buip-uahf-technical-spec.md
  +https://github.com/BitcoinUnlimited/BUIP/blob/master/BUIP-HF/buip-hf-technical-spec.md
  This is still work in progress - please check the `BUIP` repo for active
  PRs on the spec before you implement something.
  