Update DOI

Author: maksymiuks

DESCRIPTION

@@ -21,7 +21,7 @@ Description: Provides wrapper of various machine learning models.
   However, in field of the interpretable machine learning, 
   there are more and more new ideas for explaining black-box models, 
   that are implemented in 'R'. 
-  'DALEXtra' creates 'DALEX' Biecek (2018) <arXiv:1806.08915> explainer for many type of models
+  'DALEXtra' creates 'DALEX' Biecek (2018) <doi:10.48550/arXiv.1806.08915> explainer for many type of models
   including those created using 'python' 'scikit-learn' and 'keras' libraries, and 'java' 'h2o' library. 
   Important part of the package is Champion-Challenger analysis and innovative approach
   to model performance across subsets of test data presented in Funnel Plot.