Update basic-usage to reflect new default to src layout

#10135 The change in folder layout when new project is created, was not updated in the `Basic Usage` documentation. This just updates the structure with the folder layout from the CLI reference.

docs/basic-usage.md

@@ -28,8 +28,9 @@ This will create the `poetry-demo` directory with the following content:
 poetry-demo
 ├── pyproject.toml
 ├── README.md
-├── poetry_demo
-│   └── __init__.py
+├── src
+│   └── poetry_demo
+│       └── __init__.py
 └── tests
     └── __init__.py
 ```