Update tutorial code to adhere to parenthesis formatting conventions in docs

[Tbusk]

Hi, currently most of the sample code in the tutorials does not adhere to the developer guide code formatting conventions for spaces before
opening parenthesis.

https://docs.vala.dev/developer-guides/bindings/writing-a-vapi-manually/02-00-getting-started/02-07-code-formatting-conventions.html

Elementary OS also follows this convention:
https://docs.elementary.io/develop/writing-apps/code-style

And so do GNOME:
https://developer.gnome.org/documentation/guidelines/programming/coding-style.html

The documentation specifically is a bit all over the place with following that.

For example, this page has a mixture:
https://docs.vala.dev/tutorials/programming-language/main/03-00-object-oriented-programming/03-04-signals.html

And this one mostly follows it, but then doesn't.

https://docs.vala.dev/tutorials/programming-language/main/03-00-object-oriented-programming/03-07-abstract-classes.html

While this is definitely a nit-pick, I noticed it and figured I'd bring it up.