LightGBM

[docs] Fix num_leaves example (#1401)

The document says `num_leaves` should be smaller than `2^(max_depth)`.
But, its example says `70` or `80` may get better in case of `max_depth=6`.
2^6 is 64. 70 and 80 are greater than 64. The example doesn't make sense.

Changed `max_depth` from 6 to 7.