Fixed broken planetmath.org url

[ccgargantua]

I happened to notice that this URL to planetmath.org happened to be broken. I'm sure I'm the only one to have tried this in the last decade and will probably be the last one to try for the next decade, but maybe it is still worth fixing. I updated it from http://planetmath.org/encyclopedia/GoodHashTablePrimes.html to the functional https://planetmath.org/goodhashtableprimes