Clarify where to download Firebase service account JSON

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: maratal

src/pages/docs/push/getting-started/fcm.mdx

@@ -47,7 +47,7 @@ To enable push notifications, you need to configure FCM:
 4. In the Firebase Console, go to **Project Settings** → **Service accounts** and generate a new private key. Download the JSON file.
 5. In the Ably dashboard, navigate to your app's **Notifications** tab.
 6. Scroll to the **Push Notifications Setup** section and select **Configure Push**.
-7. Follow the instructions to upload your Firebase service account JSON file.
+7. Follow the instructions to upload your Firebase service account JSON file (you can download it from your Firebase console: **Project configuration** → **Service Accounts** → **Generate new private key**).
 
 ### Create an Android project <a id="prerequisites-create-project"/>