Sync totient() recipe with the standard library docs

The update step is now `n -= n // prime` which is a little simpler and faster.

```
def totient(n):
   "Count of natural numbers up to n that are coprime to n."
   # https://mathworld.wolfram.com/TotientFunction.html
   # totient(12) → 4 because len([1, 5, 7, 11]) == 4
   for prime in set(factor(n)):
       n -= n // prime
   return n
```