The LED runs from DC (direct current), not mains AC. The two wires from the mains change their polarity 50 times a second (50Hz) so they don't have a polarity (not in the usual sense of the word). Any two prong plug has no polarity, you can plug it in either way round without it making any difference to what it's connected to. Before mains AC voltage can be used to power an LED it needs to be converted to a much lower DC supply. This is handled within the housing of the LED 'bulb' unit.

Basically, doesn't matter which of the two mains wires you connect to either wire on the LED.