Update exercise 2 in xarray lecture

[gregordecristoforo]

Fix #295

Instead of inventing values for stock prices and trading volumes, the data is now downloaded via the yfinance package.

[bast]

Thanks! But then we also need to add it to environment.yml?

[gregordecristoforo]

Thanks for the reminder, I added yfinance to the environment.yml file.

[bast]

Thanks!