[CT-341] Small changie fixes

[emmyoop]

changie has been released but a few small tweaks have come up.

1. Don't repost the PR comment that a changelog entry to required for every commit pushed to the PR. Look into using https://github.com/marketplace/actions/find-comment
2. The link in that comment is wrong. It is also wrong in the header file. It should point to https://github.com/dbt-labs/dbt-core/blob/main/CONTRIBUTING.md#adding-changelog-entry
   

   dbt-core/.github/workflows/changelog-check.yml

   Line 60 in ecfd77f

   body: "Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see [the contributing guide](CONTRIBUTING.md)."

   
   

   dbt-core/.changes/header.tpl.md

   Line 6 in ecfd77f

   - Do not edit this file directly. This file is auto-generated using [changie](https://github.com/miniscruff/changie). For details on how to document a change, see [the contributing guide](CONTRIBUTING.md)