grains: clarify square numbering convention (1-based, not 0-based)
grains: clarify square numbering convention (1-based, not 0-based)

jmluy/x-common,ErikSchierboom/x-common,ErikSchierboom/x-common,jmluy/x-common,exercism/x-common,rpottsoh/x-common,jmluy/x-common,petertseng/x-common,rpottsoh/x-common,petertseng/x-common,exercism/x-common