Add "Using System;" line to default C# script

[brgillis]

This line appears in the default created script, but was missing the section showing what we should expect to see.

[raulsntos]

Thanks for contributing to the .NET documentation!

We usually don't add unused usings to code snippets, but in this case since this is the first script tutorial it makes sense to match the exact contents of the template to avoid confusing beginners.

We should also change the Complete script code snippet in line 330:

godot-docs/getting_started/step_by_step/scripting_first_script.rst

Lines 330 to 331 in 97eb080

	using Godot;