message: fix ptr in flatbuffers cpp tutorial
diff: @@ -2970,7 +2970,7 @@ We can access the type to dynamically cast the data as needed (since the
  <div class="language-cpp">
  ~~~{.cpp}
  - auto union_type = monster.equipped_type();
  + auto union_type = monster->equipped_type();
  if (union_type == Equipment_Weapon) {
  auto weapon = static_cast<const Weapon*>(monster->equipped()); // Requires `static_cast`
  