Correct exercises link in CONTRIBUTING.md (#615)
Correct exercises link in CONTRIBUTING.md (#615)

LGTM! :tada: 
exercism/x-common,jmluy/x-common,Vankog/problem-specifications,jmluy/x-common,ErikSchierboom/x-common,ErikSchierboom/x-common,petertseng/x-common,rpottsoh/x-common,kgengler/x-common,jmluy/x-common,exercism/x-common,petertseng/x-common,rpottsoh/x-common,kgengler/x-common,Vankog/problem-specifications