Update 15-6.md

[NicolasRoelandt]

Fixing 3 issues:

• it should be max, not min, since we want to maximize the conviviality
• if we take the grandchildren choice we need to add the node conviviality to the sum
• the complexity is O(n) not O(n^2) since each node is visited at most twice

[walkccc]

Thank you!