src/Doc/Tutorial/document/preface.tex

--- a/src/Doc/Tutorial/document/preface.tex	Wed Jul 18 12:21:55 2018 +0200
+++ b/src/Doc/Tutorial/document/preface.tex	Wed Jul 18 16:44:01 2018 +0200
@@ -35,7 +35,7 @@
 from output generated in this way.  The final chapter of Part~I explains how
 users may produce their own formal documents in a similar fashion.
 
-Isabelle's \hfootref{http://isabelle.in.tum.de/}{web site} contains
+Isabelle's \hfootref{https://isabelle.in.tum.de/}{web site} contains
 links to the download area and to documentation and other information.
 The classic Isabelle user interface is Proof~General~/ Emacs by David
 Aspinall's\index{Aspinall, David}.  This book says very little about