libcurl-tutorial.3: fix GnuTLS link to thread-safety guidelines

The former link was turned into a 404 at some point.

Reported-By: Askar Safin
diff --git a/docs/libcurl/libcurl-tutorial.3 b/docs/libcurl/libcurl-tutorial.3
index 17f4c3f..11f2125 100644
--- a/docs/libcurl/libcurl-tutorial.3
+++ b/docs/libcurl/libcurl-tutorial.3
@@ -271,7 +271,7 @@
 
 GnuTLS
 
- http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
+ http://gnutls.org/manual/html_node/Thread-safety.html
 
 NSS