[4.0] c890439 Fix error syntax.

[Per Buer]

commit c890439f01e916c3385f34d6c91992563b21c638
Author: Per Buer <perbu at varnish-software.com>
Date:   Thu Mar 20 11:29:57 2014 +0100

    Fix error syntax.

diff --git a/doc/sphinx/users-guide/vcl-built-in-subs.rst b/doc/sphinx/users-guide/vcl-built-in-subs.rst
index 4772374..73934c7 100644
--- a/doc/sphinx/users-guide/vcl-built-in-subs.rst
+++ b/doc/sphinx/users-guide/vcl-built-in-subs.rst
@@ -194,7 +194,7 @@ of the following keywords:
     Control will eventually pass to vcl_deliver. Caching is dependant
     on beresp.cacheable.
 
-  error [reason]
+  error(error code, reason)
     Return the specified error code to the client and abandon the request.
 
   retry