As other posters indicated, "to 5%" is most likely a typo for "by 5%".
Good news, Walk in the Light is almost a completely useless passive. Down to 5%. At this rate of a nerf every 2 days, maybe priests will only be able to pull triple our heals per second while getting buffed.
On a similar note, does blizzard even look at world of logs? Top healers for EVERY fight are priests with a few paladins yet every healer except us is getting buffed, what gives? Has Blizzard given any logic or evidence to defend this change? I don't care how you look at it, 10% mana cost increase and ~10% healing decrease is going to add up.