[DOCS] Add HTTP header example for webhook connectors (elastic#180092)

docs/management/connectors/action-types/cases-webhook.asciidoc

@@ -53,6 +53,7 @@ image::management/connectors/images/cases-webhook-connector-comments.png[Add cas
 
 Add HTTP header::
 A set of key-value pairs sent as headers with the request URLs for the create case, update case, get case, and create comment methods.
+For example, set `Content-Type` to the appropriate media type for your requests.
 
 Create case method::
 The REST API HTTP request method to create a case in the third-party system: `post`(default), `put`, or `patch`.

docs/management/connectors/action-types/webhook.asciidoc

@@ -34,7 +34,9 @@ Authentication::
 The authentication type: none, basic, or SSL.
 If you choose basic authentication, you must provide a user name and password.
 If you choose SSL authentication, you must provide SSL server certificate authentication data in a CRT and key file format or a PFX file format. You can also optionally provide a passphrase if the files are password-protected.
-HTTP headers::   A set of key-value pairs sent as headers with the request.
+HTTP headers::
+A set of key-value pairs sent as headers with the request.
+For example, set `Content-Type` to the appropriate media type for your requests.
 Certificate authority::
 A certificate authority (CA) that the connector can trust, for example to sign and validate server certificates.
 This option is available for all authentication types.