fixed typo

--- a/doc-src/IsarAdvanced/Classes/Thy/Classes.thy	Thu Feb 26 16:59:04 2009 +0100
+++ b/doc-src/IsarAdvanced/Classes/Thy/Classes.thy	Fri Feb 27 08:59:11 2009 +0100
@@ -537,7 +537,7 @@
      \end{picture}
      \caption{Subclass relationship of monoids and groups:
         before and after establishing the relationship
-        @{text "group \<subseteq> monoid"};  transitive edges left out.}
+        @{text "group \<subseteq> monoid"};  transitive edges are left out.}
      \label{fig:subclass}
    \end{center}
   \end{figure}