.github/workflows/update-terraform-providers.yml: minor fixes

- simplify comment
- move `ofborg` command into block for clickable copying

.github/workflows/update-terraform-providers.yml

@@ -28,11 +28,12 @@ jobs:
         uses: peter-evans/create-pull-request@v3
         with:
           body: |
-            Automatic update of terraform providers.
+            Automatic update by [update-terraform-providers](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/update-terraform-providers.yml) action.
 
-            Created by [update-terraform-providers](https://github.com/NixOS/nixpkgs/blob/master/.github/workflows/update-terraform-providers.yml) action.
-
-            Check that all providers build with `@ofborg build terraform-full`
+            Check that all providers build with:
+            ```
+            @ofborg build terraform-full
+            ```
           branch: terraform-providers-update
           delete-branch: false
           labels: "2.status: work-in-progress"