fix(ci): Fix the private key variable name (#395)

Author: gtema

.github/workflows/release-plz.yml

@@ -82,7 +82,7 @@ jobs:
         id: generate-token
         with:
           app-id: ${{ secrets.RELEASE_PLZ_APP_ID }}
-          private-key: ${{ secrets.RELEASE_PLZ_APP_ID }}
+          private-key: ${{ secrets.RELEASE_PLZ_PRIVATE_KEY }}
           permission-contents: write
           permission-pull-requests: write