gobject: Fix macro name in comment; improve style
Comment referenced non-existent macro: G_DEFINE_TYPE_WITH_CODE_AND_PRELUDE but it should be: _G_DEFINE_TYPE_EXTENDED_WITH_PRELUDE
Comment referenced non-existent macro: G_DEFINE_TYPE_WITH_CODE_AND_PRELUDE but it should be: _G_DEFINE_TYPE_EXTENDED_WITH_PRELUDE