Fix incorrect source url for nth-prime (#2141)

The nth-prime exercise is inspired by problem 7 at Euler, not Problem 8.
This got messed up in the http-to-https conversion in #2137.

Author: kytrinyx

exercises/nth-prime/metadata.toml

@@ -1,4 +1,4 @@
 title = "Nth Prime"
 blurb = "Given a number n, determine what the nth prime is."
 source = "A variation on Problem 7 at Project Euler"
-source_url = "https://projecteuler.net/problem=8"
+source_url = "https://projecteuler.net/problem=7"