docs: Update

docs/examples/airship.html

@@ -213,6 +213,9 @@ func drawGroundImage(screen *ebiten.Image, ground *ebiten.Image) {
                 // z is in [1, -1]
                 rate := float64(j) / float64(ph)
                 z := (1-rate)*1 + rate*-1
+                if z <= 0 {
+                        break
+                }
                 op := &ebiten.DrawImageOptions{}
                 op.GeoM.Translate(-float64(pw)/2, 0)
                 op.GeoM.Scale(1/z, 8) // 8 is an arbitrary number not to make empty lines.