trivial update to POPL title

--- a/doc-src/Ref/ref.bbl	Mon Sep 11 14:16:28 1995 +0200
+++ b/doc-src/Ref/ref.bbl	Tue Sep 12 11:04:29 1995 +0200
@@ -19,7 +19,8 @@
 \bibitem{OBJ}
 K.~Futatsugi, J.A. Goguen, Jean-Pierre Jouannaud, and J.~Meseguer.
 \newblock Principles of {OBJ2}.
-\newblock In {\em Principles of Programming Languages}, pages 52--66, 1985.
+\newblock In {\em Symposium on Principles of Programming Languages}, pages
+  52--66, 1985.
 
 \bibitem{martin-nipkow}
 Ursula Martin and Tobias Nipkow.