Fix link to HyperLogLog blogpost
Fix link to HyperLogLog blogpost

The old one gives the following error:
>Warning! Domain mapping upgrade for this domain not found. Please log in and go to the Domains Upgrades page of your blog to use this domain.
aggregateknowledge/postgresql-hll,citusdata/postgresql-hll,aggregateknowledge/postgresql-hll,citusdata/postgresql-hll