sum-of-multiples: update description (#476)
sum-of-multiples: update description (#476)

description updated to mention limits,
https://github.com/exercism/xgo/issues/340#issuecomment-238150900
ErikSchierboom/x-common,jmluy/x-common,exercism/x-common,petertseng/x-common,Vankog/problem-specifications,rpottsoh/x-common,jmluy/x-common,jmluy/x-common,exercism/x-common,rpottsoh/x-common,kgengler/x-common,kgengler/x-common,Vankog/problem-specifications,petertseng/x-common,ErikSchierboom/x-common