Update dominoes canonical-data expected key (#823)
Update dominoes canonical-data expected key (#823)

It was "can-chain", but most other exercises have "expected" as the expected output of a property being tested.
exercism/x-common,kgengler/x-common,petertseng/x-common,ErikSchierboom/x-common,exercism/x-common,jmluy/x-common,rpottsoh/x-common,kgengler/x-common,jmluy/x-common,rpottsoh/x-common,Vankog/problem-specifications,ErikSchierboom/x-common,petertseng/x-common,Vankog/problem-specifications,jmluy/x-common