devel/libavl: update WWW

Approved by:	portmgr blanket

devel/libavl/pkg-descr

@@ -4,4 +4,4 @@ search operations. libavl is the most complete free balanced binary tree
 library on the net today, supporting AVL trees in plain, threaded, and
 right-threaded forms as well as plain red-black trees.
 
-WWW: http://www.stanford.edu/~blp/avl/
+WWW: https://adtinfo.org/