fix cpp snippet

Author: Wumpf

docs/snippets/all/descriptors/descr_builtin_component.cpp

@@ -6,7 +6,10 @@ int main() {
 
     rec.log_static(
         "data",
-        rerun::ComponentBatch::from_loggable(rerun::Position3D(1.0f, 2.0f, 3.0f))
+        rerun::ComponentBatch::from_loggable(
+            rerun::Position3D(1.0f, 2.0f, 3.0f),
+            rerun::Loggable<rerun::Position3D>::Descriptor
+        )
     );
 
     // The tags are indirectly checked by the Rust version (have a look over there for more info).