Apply suggestions from code review

Co-authored-by: Danil Alexeev <dalexeev12@yandex.ru>

Author: aaronfranke

modules/gdscript/gdscript_parser.cpp

@@ -684,7 +684,7 @@ void GDScriptParser::parse_program() {
 				}
 				advance();
 				if (has_early_abstract) {
-					push_error(R"(Expected "class_name" or "class" after "abstract".)");
+					push_error(R"(Expected "class_name", "extends", or "class" after "abstract".)");
 				} else {
 					has_early_abstract = true;
 				}
@@ -6315,7 +6315,7 @@ void GDScriptParser::TreePrinter::print_tree(const GDScriptParser &p_parser) {
 		push_line("@tool");
 	}
 	if (class_tree->is_abstract) {
-		push_line("@abstract");
+		push_line("abstract");
 	}
 	if (!class_tree->icon_path.is_empty()) {
 		push_text(R"(@icon (")");

modules/gdscript/tests/scripts/analyzer/features/extend_abstract_class.gd

@@ -8,8 +8,7 @@ class B extends A:
 class C extends CanvasItem:
 	pass
 
-abstract
-class X:
+abstract class X:
 	pass
 
 class Y extends X: