docs: Add import path for `delete_old_tasks()`

[albertocerrone]

Why? 🤔

I was reading the documentation to understand how to add garbage collection for old tasks in my project and I had to spend a few minutes to find from where to import delete_old_tasks()

What? 🤌🏼

Updated the README with the import path for delete_old_tasks()

[Bogdanp]

Thanks!