fixed typo in NavigationPolygon doc

(cherry picked from commit 30d22305e10d66ec661b06db3b6b047143a35bfe)

doc/classes/NavigationPolygon.xml

@@ -18,7 +18,7 @@
 		var polygon = NavigationPolygon.new()
 		var vertices = PoolVector2Array([Vector2(0, 0), Vector2(0, 50), Vector2(50, 50), Vector2(50, 0)])
 		polygon.set_vertices(vertices)
-		var indices = PoolIntArray(0, 3, 1)
+		var indices = PoolIntArray([0, 1, 2, 3])
 		polygon.add_polygon(indices)
 		$NavigationPolygonInstance.navpoly = polygon
 		[/codeblock]