fix(webhook): clarify endpoint expects application/json payloads (#13786)

This PR updates the subtitle under the "Endpoint URL" input on the
webhook creation form.

The updated text clearly states that the server sends POST requests with
application/json and recommends setting the correct Content-Type.

This addresses issue #13757.

Closes #13757

---------

Co-authored-by: Charles Bochet <charlesBochet@users.noreply.github.com>
Co-authored-by: ehconitin <nitinkoche03@gmail.com>

nx.json

@@ -263,5 +263,5 @@
     "default": { "options": { "cacheableOperations": ["storybook:build"] } }
   },
   "useInferencePlugins": false,
-  "defaultBase": "main",
+  "defaultBase": "main"
 }