fixed bib

close #327

user-doc/bibliography.bib

@@ -2698,7 +2698,6 @@ pages = {3999--4000},
 	Pages = {773--781},
 	Publisher = {Curran Associates, Inc., Red Hook, NY},
 	Title = {Non-strongly-convex smooth stochastic approximation with convergence rate O(1/n)},
-        Url = {http://papers.nips.cc/paper/4900-non-strongly-convex-smooth-stochastic-approximation-with-convergence-rate-o1n.pdf}
 	Year = {2013}
 }