Fix minor grammar issue s/the/a/
Fix minor grammar issue s/the/a/

As suggested in: https://github.com/exercism/x-common/pull/307#discussion_r73289379

ErikSchierboom/x-common,Vankog/problem-specifications,ErikSchierboom/x-common,rpottsoh/x-common,kgengler/x-common,petertseng/x-common,petertseng/x-common,rpottsoh/x-common,kgengler/x-common,Vankog/problem-specifications,jmluy/x-common,jmluy/x-common,exercism/x-common,jmluy/x-common,exercism/x-common