README.md: grammar corrections
README.md: grammar corrections

Minor grammatical corrections.
rpottsoh/x-common,jmluy/x-common,jmluy/x-common,ErikSchierboom/x-common,exercism/x-common,petertseng/x-common,petertseng/x-common,jmluy/x-common,rpottsoh/x-common,ErikSchierboom/x-common,exercism/x-common