Fix Nielsen's severity ratings link on assign #4
Fix Nielsen's severity ratings link on assign #4

Former-commit-id: a156d452347e937681b43f601d1443e01881d296
Former-commit-id: d9b83fd64b0db0aeb9121338a2441ee10bb906fb
Former-commit-id: 31e1dc07e466da0d8e7cd2e244d5a1cff729fa79
StanfordHCI/cs247,StanfordHCI/cs247,StanfordHCI/cs247