[BUG] in sample: delete-unused-accounts-cron TypeError: PromisePool is not a constructor

### Which sample has a bug?

delete-unused-accounts-cron

### How to reproduce the issue

I've deployed the sample function exactly per instructions in readme, however, the function fails in firebase and the logs returns:

accountcleanup
TypeError: PromisePool is not a constructor

here is the line (33) of code that triggers the issue:

const promisePool = new PromisePool(() => deleteInactiveUser(inactiveUsers), MAX_CONCURRENT);