Add reference to template dir in the tutorial

recipes/001_first.py

@@ -184,6 +184,7 @@ def plot_optimizaiton_history_matplotlib(study: optuna.study.Study) -> plt.Figur
 # You need to create a pull request to the `optunahub-registry <https://github.com/optuna/optunahub-registry>`_ repository.
 #
 # The following is an example of the directory structure of the pull request.
+# See the `template directory<https://github.com/optuna/optunahub-registry/tree/main/template>`_ for an example of the directory structure.
 #
 # | package
 # | └── samplers (or visualization)