hello-world: Rename language to track ID
hello-world: Rename language to track ID

In Exercism "language" refers to the programming language name, (e.g.
C++), whereas Track ID refers to the identifier used on Exercism (e.g.
cpp).

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