fixed RALib ref

sigproc.bib

@@ -222,7 +222,8 @@ machine learning algorithms},
 	shorttitle = {{RALib}},
 	urldate = {2016-09-05},
 	booktitle = {{DIFTS}},
-	author = {Cassel, Sofia and Howar, Falk and Jonsson, Bengt},
+	author = {Cassel, S. and Howar, F. and Jonsson, B.},
+	url = {http://www.faculty.ece.vt.edu/chaowang/difts2015/papers/paper_5.pdf},
 	year = {2015},
 	keywords = {learning, ralib, register-automata},
 }