What is the name of the tournament played on Sep 20, 1981, ending with a margin of vicroty of 2 strokes?
CREATE TABLE "pga_tour_wins_16" (
  "date" text,
  "tournament" text,
  "winning_score" text,
  "margin_of_victory" text,
  "runner_s_up" text
);
SELECT "tournament" FROM "pga_tour_wins_16" WHERE "margin_of_victory"='2 strokes' AND "date"='sep 20, 1981';