Update mobile-services-filter-user-results-dotnet-backend.md

If you put the code after "TodoItem current = await InsertAsync(item);"
The UserId column in the database will be NULL.
So I think it should be   
>"4. Locate the **PostTodoItem** method and add the following code at the end right before the **TodoItem 
current = await InsertAsync(item);** statement:" 

instead of 
>"4. Locate the **PostTodoItem** method and add the following code at the end right before the **return** statement:"