Update Part-9.md

Fix typo

docs/en/Tutorials/Part-9.md

@@ -90,7 +90,7 @@ Create a new razor page, `Index.cshtml` under the `Pages/Authors` folder of the
 
 This is a simple page similar to the Books page we had created before. It imports a JavaScript file which will be introduced below.
 
-### IndexModel.cshtml.cs
+### Index.cshtml.cs
 
 ````csharp
 using Microsoft.AspNetCore.Mvc.RazorPages;