fix schema error in contentType

a contentType may be a comma-separated list of media-ranges, not just a single media-range

Author: karenetheridge

src/schemas/validation/schema.yaml

@@ -618,8 +618,8 @@ $defs:
     type: object
     properties:
       contentType:
+        $comment: one or more comma-separated media-ranges
         type: string
-        format: media-range
       headers:
         type: object
         propertyNames: