Remove unnecessary namespace specification.

docs/en/Tutorials/Part-4.md

@@ -126,7 +126,7 @@ public async Task Should_Create_A_Valid_Book()
         {
             Name = "New test book 42",
             Price = 10,
-            PublishDate = System.DateTime.Now,
+            PublishDate = DateTime.Now,
             Type = BookType.ScienceFiction
         }
     );
@@ -208,7 +208,7 @@ namespace Acme.BookStore.Books
                 {
                     Name = "New test book 42",
                     Price = 10,
-                    PublishDate = System.DateTime.Now,
+                    PublishDate = DateTime.Now,
                     Type = BookType.ScienceFiction
                 }
             );