Remove --checkout 3.0 in the extension tutorial

docs/source/extension/extension_tutorial.rst

@@ -101,7 +101,7 @@ This will create a new folder for your extension in your current directory.
 
 .. code:: bash
 
-    cookiecutter https://github.com/jupyterlab/extension-cookiecutter-ts --checkout 3.0
+    cookiecutter https://github.com/jupyterlab/extension-cookiecutter-ts
 
 When prompted, enter values like the following for all of the cookiecutter
 prompts (``apod`` stands for Astronomy Picture of the Day, the NASA service we