[HANDS-ON BUG] Unable to start virtual screen on unit3

# Describe the bug

The unit3 notebook is missing the reload session code in order to activate python virtual screen.

# Material

- On google scholar

# Step to reproduce

- load the [notebook](https://colab.research.google.com/github/huggingface/deep-rl-class/blob/main/notebooks/unit3/unit3.ipynb) and execute code until cell 6, it will crash telling xvfb folder is missing

# How to fix

Add the same code as notebook 1 and 2 just below #scrollTo=jV6wjQ7Be7p5
```python
import os
os.kill(os.getpid(), 9)
```
To fix