Change W1ex19 test to better reflect the exercise text

The exercise reads:

> implement a function nextPrime that returns the first prime
> number that comes after the given number. Use the function isPrime
> you just defined.

Since it says "first prime number that comes after the given number", I take that to mean that if the number given is prime, it should not return that number but the next prime number. When the test here is (<n), my implementation breaks for (nextPrime 79). Since 79 is prime, the function returns 83, the next prime number, but the test claims that 79 should be returned.