fixed typo

--- a/src/Pure/context.ML	Wed Jun 07 02:04:20 2006 +0200
+++ b/src/Pure/context.ML	Wed Jun 07 16:53:31 2006 +0200
@@ -518,7 +518,7 @@
 
 fun transfer_proof thy' (prf as Proof (thy_ref, data)) =
   if not (subthy (deref thy_ref, thy')) then
-    error "transfer proof context: no a super theory"
+    error "transfer proof context: not a super theory"
   else Proof (self_ref thy', data);