tutorial still uses old name for open3d tutorial example script

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Describe the issue

This is a tutorial documentation issue that prevents the tutorial from working without significant user debugging.

This document

http://www.open3d.org/docs/release/tutorial/t_geometry/pointcloud.html

says to

`import open3d_tutorial as o3dtut
`

but that will never be found, because the file was renamed in a prior issue and is now

`open3d_example.py
`

which may also not be found because it doesn't live in any default path.

If the document were to say this instead, it will likely prevent people from having to go through layers of searches and old, broken solutions (like the one currently on StackOverflow):

```
sys.path.append(os.path.expanduser('~/.local/lib/python3.10/site-packages/open3d/examples'))
import open3d_example as o3dtut
```

### Steps to reproduce the bug

```python
n/a
```

### Error message

_No response_

### Expected behavior

_No response_

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 20.04 / macOS 10.15 / Windows 10 64-bit
- Python version: Python 3.8 / output from `import sys; print(sys.version)`
- Open3D version: output from python: `print(open3d.__version__)`
- System architecture: x86 / arm64 / apple-silicon / jetson / rpi
- Is this a remote workstation?: yes or no
- How did you install Open3D?: pip / conda / build from source
- Compiler version (if built from source): gcc 7.5 / clang 7.0
```

### Additional information

_No response_