Improve backend update description

.github/workflows/update_backend.yml

@@ -60,3 +60,13 @@ jobs:
 
       - name: "Create PR"
         uses: peter-evans/create-pull-request@v3
+        env:
+          GITHUB_TOKEN: ${{ secrets.REPO_SCOPED_TOKEN }}
+          title: "Automated update to backend: v${{ steps.get-latest.outputs.version }}"
+          body: |
+            This PR was automatically generated by the GitHub Action
+            [PySR backend update](.github/workflows/update-backend.yml).
+
+            It updates the backend version to v${{ steps.get-latest.outputs.version }}.
+            For a full description of the changes, see the backend
+            changelog: [v${{ steps.get-latest.outputs.version }}](https://github.com/MilesCranmer/SymbolicRegression.jl/releases/tag/v${{ steps.get-latest.outputs.version }}).