It's the Mont du Lac des Cygnes in the Parc National des Grands Jardins.
If I'm correct, and my memories aren't wrong , it's around those coordinates: 47°39'55.9"N 70°36'11.8"W. It's right behind a security fence that, for obvious reasons of it being a security fence, you should not cross ;-) But hey, for a great picture!
I would advice the adventure of going there really early in the morning, at the right time of the year, the sun rises right in fron of your when you're there and the sight is amazing!
While you're around, if you want to hike, my heart goes to the Acropole des Draveurs in the Parc National des Hautes-Gorges-de-La-Rivière-Malbaie. The sights are to die for (it's the kind you head in the header picture of above linked site), and the feeling of changing biome from a leafy forest to a tundra is one of a kind!