Update PygameIntro.rst

if you write black on screen.fill(black) line its shows an arrow screen.fill("black") it worked like this in my computer

docs/reST/tut/PygameIntro.rst

@@ -89,7 +89,7 @@ along, and a complete breakdown follows.
        if ballrect.top < 0 or ballrect.bottom > height:
            speed[1] = -speed[1]
        
-       screen.fill(black)
+       screen.fill("black")
        screen.blit(ball, ballrect)
        pygame.display.flip()