Help change to conform to Toolbox change.

org.lamport.tla.toolbox.doc/html/spec/spec.html

@@ -48,11 +48,7 @@ help page.
 </p>
 
  <p>
-You can run multiple instances of the Toolbox to work on different
-specifications concurrently.&nbsp; 
-
-Do not open the same specification in two different instances of the Toolbox
-at the same time.
+You can run only one instance of the Toolbox at a time.&nbsp; 
  </p>
 <hr> 
 <dl>