Update rbo.py

[DnlRKorn]

The previous link https://researcher.watson.ibm.com/researcher/files/us-fagin/topk.pdf is now dead; I replaced it with a link to the journal paper and added citations for both papers for accessibility in the future (in case the links go down in some other way)

[canderson-luminatedata]

@changyaochen @DnlRKorn I was going to create a similar PR. This is a useful addition.