bench: Update span-serialize comment

Commit faecca9a85 changed the type of
block413567 from vector to span, but forgot to update the comment. Do it
now.

src/bench/load_external.cpp

@@ -44,8 +44,7 @@ static void LoadExternalBlockFile(benchmark::Bench& bench)
     auto params{testing_setup->m_node.chainman->GetParams()};
     ss << params.MessageStart();
     ss << static_cast<uint32_t>(benchmark::data::block413567.size());
-    // We can't use the streaming serialization (ss << benchmark::data::block413567)
+    // Use span-serialization to avoid writing the size first.
-    // because that first writes a compact size.
     ss << std::span{benchmark::data::block413567};
 
     // Create the test file.