update DOI format (note on R-hub)

Author: maelle

DESCRIPTION

@@ -19,7 +19,7 @@ Description: The Datasaurus Dozen is a set of datasets with the same summary sta
              is generated in a novel way. The simulated annealing process used to derive datasets 
              from the original Datasaurus is detailed in "Same Stats, Different Graphs: Generating 
              Datasets with Varied Appearance and Identical Statistics through Simulated Annealing" 
-             <http://dx.doi.org/10.1145/3025453.3025912>.
+             <doi:10.1145/3025453.3025912>.
 Depends: R (>= 3.0.0)
 License: MIT + file LICENSE
 Encoding: UTF-8