Updated sample to use new main config style

Tutorial.md

@@ -17,7 +17,8 @@ The rest of this tutorial will assume that the CATCH single header or root folde
 Let's start with a really simple example. Say you have written a function to calculate factorials and now you want to test it (let's leave aside TDD for now). To keep things simple we'll put everything in a single file.
 
 ```c++
-    #include "catch_with_main.hpp" // This brings in a default implementation for main()
+    #define CATCH_CONFIG_MAIN  // This tell CATCH to provide a main() - only do this in one cpp file
+    #include "catch.hpp"
 
     unsigned int Factorial( unsigned int number )
     {