Add content-type to Generic Alert docs

Since Generic Alert Endpoint expects JSON as a payload
it should have a "Content-Type: application/json"

doc/user/project/integrations/generic_alerts.md

@@ -35,7 +35,11 @@ You can customize the payload by sending the following parameters. All fields ar
 Example request:
 
 ```sh
-curl --request POST --data '{"title": "Incident title"}' --header "Authorization: Bearer <autorization_key>" <url>
+curl --request POST \
+  --data '{"title": "Incident title"}' \
+  --header "Authorization: Bearer <autorization_key>" \
+  --header "Content-Type: application/json" \
+  <url>
 ```
 
 The `<autorization_key>` and `<url>` values can be found when [setting up generic alerts](#setting-up-generic-alerts).