Update downloads.md to correct release date
Update downloads.md to correct release date

Correct release date for 2.16.0.
lukecwik/incubator-beam,apache/beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,iemejia/incubator-beam,lukecwik/incubator-beam,RyanSkraba/beam,lukecwik/incubator-beam,chamikaramj/beam,chamikaramj/beam,RyanSkraba/beam,apache/beam,chamikaramj/beam,RyanSkraba/beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,RyanSkraba/beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,iemejia/incubator-beam,RyanSkraba/beam,robertwb/incubator-beam,RyanSkraba/beam,RyanSkraba/beam,robertwb/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,chamikaramj/beam,apache/beam,robertwb/incubator-beam,lukecwik/incubator-beam