Stanford University is sore. Or, at any rate, its sports publicity man is. Bob Murphy says he is tired of the artificial-grass people yakking all the time about how much safer their stuff is than the real thing ( Stanford uses plain old grass in its stadium). He claims that Stanford's football team had only one knee injury all last season, and that was in an away game on artificial turf. "What really burns us," Murphy cries, "is the way some coaches will tell the mothers of kids they are trying to recruit, 'You wouldn't want your boy to break his leg playing on ordinary grass, would you?' "