fix: Use full path to cortex-mixin.zip artifact

Co-authored-by: Marco Pracucci <[email protected]>

Author: jdbaldry

RELEASE.md

@@ -26,7 +26,7 @@ A release of `cortex-jsonnet` should follow shortly after a release of Cortex.
     ```console
    $ make build-mixin
    ```
-7. Add the `cortex-mixin.zip` and release change log to the GitHub release.
+7. Add the `cortex-mixin/cortex-mixin.zip` and release change log to the GitHub release.
    - Edit the release in GitHub by going to https://github.com/grafana/cortex-jsonnet/releases/edit/x.y.z
 
 ### How to tag a release