Fix code reference (#4872)

Fixes #4871

Author: roji

samples/core/Miscellaneous/NewInEFCore9/QuerySample.cs

@@ -215,17 +215,17 @@ async Task<List<Post>> GetPostsForceConstantCollection(int[] ids)
         Console.WriteLine();
         Console.WriteLine("Aggregate over subquery/aggregate:");
         Console.WriteLine();
-        // </AggregateOverSubquery>
-        var latestPostsAverageRatingByLanguage = await context.Blogs.
-            Select(x => new
+        #region AggregateOverSubquery
+        var latestPostsAverageRatingByLanguage = await context.Blogs
+            .Select(x => new
             {
                 x.Language,
-                LatestPostRating = x.Posts.OrderByDescending(xx => xx.PublishedOn).FirstOrDefault().Rating
+                LatestPostRating = x.Posts.OrderByDescending(xx => xx.PublishedOn).FirstOrDefault()!.Rating
             })
             .GroupBy(x => x.Language)
             .Select(x => x.Average(xx => xx.LatestPostRating))
             .ToListAsync();
-        // </AggregateOverSubquery>
+        #endregion
 
         // Max over decimal is not supported on Sqlite
         if (!context.UseSqlite)