samples: counter: maxim_ds3231: Fix outdated link

This commit fixes the outdated link to the "Chronodot" product page.

It seems "macetech" no longer lists the older versions of this product
with DS3231, so the link has been replaced with the one from Adafruit.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>