message: minor: Fix incorrect call to insert_one() in tutorial.md
diff: @@ -212,7 +212,8 @@ To insert a single document into the collection, use a
  instance's `insert_one()` method:
  ```c++
  -mongocxx::result::insert_one result = restaurants.insert_one(doc);
  +mongocxx::stdx::optional<mongocxx::result::insert_one> result =
  + restaurants.insert_one(doc);
  ```
  If you do not specify a top-level `_id` field in the document,
  