Update extended_similarity_indices.py

 - fix attribution statement
 - update citations to include both papers

aimsim/tasks/extended_similarity_indices.py

@@ -1,10 +1,13 @@
 """
 Calculates the Extended Similarity Indexes as shown in
 this table: https://jcheminf.biomedcentral.com/articles/10.1186/s13321-021-00505-3/tables/1
-and described in "Extended similarity indices: the benefits of comparing more than two objects
-simultaneously. Part 1: Theory and characteristics"
+and described in:
+ - Extended similarity indices: the benefits of comparing more than two objects simultaneously. Part 1: Theory and characteristics
+   - https://doi.org/10.1186/s13321-021-00505-3
+ - Extended similarity indices: the benefits of comparing more than two objects simultaneously. Part 2: speed, consistency, diversity selection
+   - https://link.springer.com/article/10.1186/s13321-021-00504-4
 
-Both gen_sim_dict and calculate_counters were provided by Raymond Quintana, similar to
+Both gen_sim_dict and calculate_counters were provided by Ramon Alain Miranda Quintana, similar to
 that which is here: https://github.com/ramirandaq/MultipleComparisons
 """
 import json