Dead link in FastTreeRegressionTrainerClass Documentation

The link to an article describing greedy function evaluation is broken: https://projecteuclid.org/DPubS?service=UI&version=1.0&verb=Display&handle=euclid.aos/1013203451.

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#### Document Details

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* ID: 9f7b4cf1-c7bf-8141-180b-ea4474964153
* Version Independent ID: 6ba65d38-7383-fdfb-ada4-12caf50f71ab
* Content: [FastTreeRegressionTrainer Class (Microsoft.ML.Trainers.FastTree)](https://docs.microsoft.com/en-us/dotnet/api/microsoft.ml.trainers.fasttree.fasttreeregressiontrainer?view=ml-dotnet)
* Content Source: [dotnet/xml/Microsoft.ML.Trainers.FastTree/FastTreeRegressionTrainer.xml](https://github.com/dotnet/ml-api-docs/blob/live/dotnet/xml/Microsoft.ML.Trainers.FastTree/FastTreeRegressionTrainer.xml)
* Product: **dotnet-ml-api**
* GitHub Login: @natke
* Microsoft Alias: **nakersha**