grade-school.md: clarification on mutability
grade-school.md: clarification on mutability

This bonus exercise doesn't apply to e.g. Elixir with its immutable data structures.
exercism/x-common,wobh/x-common,petertseng/x-common,Vankog/problem-specifications,ErikSchierboom/x-common,jmluy/x-common,kgengler/x-common,rpottsoh/x-common,bennn/x-common,petertseng/x-common,exercism/x-common,ErikSchierboom/x-common,ekse/exercism-common,kgengler/x-common,Vankog/problem-specifications,rpottsoh/x-common,jmluy/x-common,jmluy/x-common