docs: Update

docs/examples/airship.html

@@ -210,12 +210,12 @@ func drawGroundImage(screen *ebiten.Image, ground *ebiten.Image) {
         gw, _ := ground.Size()
         pw, ph := perspectiveGroundImage.Size()
         for j := 0; j < ph; j++ {
-                // z is in [1.5, 0.01]
+                // z is in [1, -1]
                 rate := float64(j) / float64(ph)
-                z := (1-rate)*1.5 + rate*0.01
+                z := (1-rate)*1 + rate*-1
                 op := &ebiten.DrawImageOptions{}
                 op.GeoM.Translate(-float64(pw)/2, 0)
-                op.GeoM.Scale(1/z, 4) // 4 is an arbitrary number not to make empty lines.
+                op.GeoM.Scale(1/z, 8) // 8 is an arbitrary number not to make empty lines.
                 op.GeoM.Translate(float64(pw)/2, float64(j)/z)
 
                 src := image.Rect(0, j, gw, j+1)