To the Editor:I would be most grateful to you if you would get off Senator Kor-shak’s back—just for a little while.When we beat our strategic retreat to Winnetka a couple of years ago, we clung to our Herald subscription as sort of an umbilical cord to the world of Right and Wrong. The trouble is that in the interim, Mrs. Petrie has learned about all sorts of grey shadesthat make her hard to keep in line.Your editorial of August 19 ( Dear Marshall, even when you’re right, you’re wrong) almost tore it.So do me a favor, and stay off Marshall for a while. Otherwise she’ll cancel our subscription. After all, how many readers do you have north of Kenilworth?Donald A. Petrie