Updated Art of Prolog links to use more stable MIT URLs

Author: chuckhoupt

docs/_data/sterling86.yml

@@ -2,7 +2,7 @@ cite:
   short: Sterling 86
   long: The Art of Prolog, Leon S. Sterling and Ehud Y. Shapiro, MIT Press, 1986
 
-url: https://www.dropbox.com/s/cvooh6nt83ajng2/1407.pdf?dl=0&page=
+url: https://mitp-content-server.mit.edu/books/content/sectbyfn/books_pres_0/1407/1407.pdf#page=
 
 sec2page:
   "4.1"  :  92