fix errors introduced by earlier PR (#1027)

Author: tdykstra

samples/modelbuilder/MatrixFactorization_MovieRecommendation/MovieRecommender_WebApi/Program.cs

@@ -1,6 +1,7 @@
 // This file was auto-generated by ML.NET Model Builder. 
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.ML;
 using Microsoft.OpenApi.Models;
 using System.Threading.Tasks;