docs: address review feedback on notifications docs

Use dynamic import pattern for SubPageBlueprint loader instead of
Promise.resolve, and fix missing comma in TypeScript example.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

docs/notifications/usage.md

@@ -232,7 +232,7 @@ Below are metadata fields that will be commonly used between processors and have
 # Example:
 const payload = {
   title: 'Entities Require Attention',
-  description: 'Entities: Service A, Service B'
+  description: 'Entities: Service A, Service B',
   metadata: {
      'backstage.io/body.markdown': `
         # Entities