Fix sub-aggregation example

[gmarz]

Originally reported here http://stackoverflow.com/questions/28153597/elasticsearch-net-nest-sub-aggregation

http://nest.azurewebsites.net/nest/aggregations/handling.html

The sub-aggregation example should instead be:

var terms = result.Aggs.Terms("my_agg");

foreach(var term in terms.Items)
{
    var max = term.Max("my_sub_agg");
}