Fix /listings/id endpoint

Return listing object directly instead of wrapping
it in a list.

Signed-off-by: Carsten Reckord <reckord@yatta.de>

src/main/java/org/eclipsefoundation/marketplace/resource/ListingResource.java

@@ -140,7 +140,7 @@ public class ListingResource {
 			throw new NoResultException("Could not find any documents with ID " + listingId);
 		}
 		// return the results as a response
-		return responseBuider.build(listingId, params, cachedResults.get());
+		return responseBuider.build(listingId, params, cachedResults.get().get(0));
 	}
 
 	/**