Fix PATCH /cart/{identifier} modified by mistake

Author: adriendupuis

docs/api/rest_api/rest_api_reference/input/ibexa-cart.raml

@@ -88,7 +88,7 @@ post:
         displayName: Update Cart's metadata
         description: Updates identified Cart's metadata from payload.
         headers:
-            Accept:
+            Content-Type:
                 description: The CartMetadataUpdate schema encoded in XML or JSON format.
                 example: |
                     application/vnd.ibexa.api.CartMetadataUpdate+xml