Add --headless parameter to "compiling with dotnet" examples

[adamscott]

We already tell users to use --headless to generate the glue. This PR make it so that we use --headless in the examples.

godot-docs/contributing/development/compiling/compiling_with_dotnet.rst

Lines 28 to 38 in 9212f4f

	Generate the glue
	-----------------
	Parts of the sources of the managed libraries are generated from the ClassDB.
	These source files must be generated before building the managed libraries.
	They can be generated by any .NET-enabled Godot editor binary by running it with
	the parameters ``--headless --generate-mono-glue`` followed by the path to an
	output directory.
	This path must be ``modules/mono/glue`` in the Godot directory::
	<godot_binary> --headless --generate-mono-glue modules/mono/glue

[AThousandShips]

Makes sense, and matches what we do in CI so works well

[mhilbrunner]

Thanks! Merged. :)