message: fix cpp usage markdown error
diff: @@ -117,8 +117,8 @@ To use:
  GetMonster(flatbuffer)->UnPackTo(&monsterobj);
  // Update object directly like a C++ class instance.
  - cout << monsterobj->name; // This is now a std::string!
  - monsterobj->name = "Bob"; // Change the name.
  + cout << monsterobj.name; // This is now a std::string!
  + monsterobj.name = "Bob"; // Change the name.
  // Serialize into new flatbuffer.
  FlatBufferBuilder fbb;
  