trivial update to reference

--- a/doc-src/Logics/logics.bbl	Wed Oct 19 09:23:56 1994 +0100
+++ b/doc-src/Logics/logics.bbl	Wed Oct 19 09:39:23 1994 +0100
@@ -140,7 +140,7 @@
 \newblock {\em Theoretical Computer Science}, 87:209--220, 1991.
 
 \bibitem{noel}
-Philippe {No\"el}.
+Philippe No{\"e}l.
 \newblock Experimenting with {Isabelle} in {ZF} set theory.
 \newblock {\em Journal of Automated Reasoning}, 10(1):15--58, 1993.