vnc: Fix spelling (hellmen -> hellman) in comment

The algorithm was named after Martin E. Hellman.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

ui/vnc-tls.c

@@ -49,7 +49,7 @@ static int vnc_tls_initialize(void)
     if (gnutls_global_init () < 0)
         return 0;
 
-    /* XXX ought to re-generate diffie-hellmen params periodically */
+    /* XXX ought to re-generate diffie-hellman params periodically */
     if (gnutls_dh_params_init (&dh_params) < 0)
         return 0;
     if (gnutls_dh_params_generate2 (dh_params, DH_BITS) < 0)