Fix `ADD_PROPERTY()` calls in GDExtension C++ example to use correct syntax

This matches how properties are declared in core Godot.

tutorials/scripting/gdextension/gdextension_cpp_example.rst

@@ -497,6 +497,7 @@ show the methods we end up changing, don't remove the lines we're omitting:
     void GDExample::_bind_methods() {
         ClassDB::bind_method(D_METHOD("get_amplitude"), &GDExample::get_amplitude);
         ClassDB::bind_method(D_METHOD("set_amplitude", "p_amplitude"), &GDExample::set_amplitude);
+
         ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "amplitude"), "set_amplitude", "get_amplitude");
     }
 
@@ -554,7 +555,8 @@ showing the methods that have changed so don't remove anything we're omitting:
         ...
         ClassDB::bind_method(D_METHOD("get_speed"), &GDExample::get_speed);
         ClassDB::bind_method(D_METHOD("set_speed", "p_speed"), &GDExample::set_speed);
-        ADD_PROPERTY("GDExample", PropertyInfo(Variant::FLOAT, "speed", PROPERTY_HINT_RANGE, "0,20,0.01"), "set_speed", "get_speed");
+
+        ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "speed", PROPERTY_HINT_RANGE, "0,20,0.01"), "set_speed", "get_speed");
     }
 
     GDExample::GDExample() {
@@ -648,7 +650,7 @@ as follows:
 
     void GDExample::_bind_methods() {
         ...
-        ADD_PROPERTY("GDExample", PropertyInfo(Variant::FLOAT, "speed", PROPERTY_HINT_RANGE, "0,20,0.01"), "set_speed", "get_speed");
+        ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "speed", PROPERTY_HINT_RANGE, "0,20,0.01"), "set_speed", "get_speed");
 
         ADD_SIGNAL(MethodInfo("position_changed", PropertyInfo(Variant::OBJECT, "node"), PropertyInfo(Variant::VECTOR2, "new_pos")));
     }