avoid theory Imperative_HOL altogether

--- a/doc-src/Codegen/Thy/Further.thy	Thu Sep 02 16:41:41 2010 +0200
+++ b/doc-src/Codegen/Thy/Further.thy	Thu Sep 02 16:41:42 2010 +0200
@@ -122,7 +122,7 @@
   specific application, you should consider \emph{Imperative
   Functional Programming with Isabelle/HOL}
   \cite{bulwahn-et-al:2008:imperative}; the framework described there
-  is available in session @{theory Imperative_HOL}.
+  is available in session @{text Imperative_HOL}.
 *}