Update robust_single_linkage_.py
Update robust_single_linkage_.py

Fix robust single linkage.
brunoalano/hdbscan,lmcinnes/hdbscan,scikit-learn-contrib/hdbscan,c-north/hdbscan,scikit-learn-contrib/hdbscan,lmcinnes/hdbscan,brunoalano/hdbscan