Update shebang line for hello-world.cs (#49923)

Changed the file-based apps `hello-world.cs` snipped to use a more widely usable shebang line:

`#!/usr/bin/env dotnet`

This should work on most *nix systems (including macOS) assuming `dotnet` is on the path.

Author: DamianEdwards

docs/csharp/fundamentals/program-structure/snippets/file-based-program/hello-world.cs

@@ -1,2 +1,2 @@
-#!/usr/local/share/dotnet/dotnet run
+#!/usr/bin/env dotnet
 Console.WriteLine("Hello, World!");