Fixed broken link to polygon algorithm article

Author: GeospatialPython

shapefile.py

@@ -96,7 +96,8 @@ def __repr__(self):
 
 def signed_area(coords):
     """Return the signed area enclosed by a ring using the linear time
-    algorithm at http://www.cgafaq.info/wiki/Polygon_Area. A value >= 0
+    algorithm at:
+    https://web.archive.org/web/20080209143651/http://cgafaq.info/wiki/Polygon_Area
     indicates a counter-clockwise oriented ring.
     """
     xs, ys = map(list, zip(*coords))