updated example;

--- a/src/Doc/System/Sessions.thy	Sat Nov 16 12:41:16 2013 +0100
+++ b/src/Doc/System/Sessions.thy	Sat Nov 16 13:07:38 2013 +0100
@@ -399,7 +399,7 @@
   \smallskip Build some session images with cleanup of their
   descendants, while retaining their ancestry:
 \begin{ttbox}
-isabelle build -b -c HOL-Boogie HOL-SPARK
+isabelle build -b -c HOL-Algebra HOL-Word
 \end{ttbox}
 
   \smallskip Clean all sessions without building anything: