doc: Fix threading documentation.

doc/threading.dox

@@ -25,7 +25,7 @@ use, using ssh_threads_set_callbacks(), then call ssh_init().
 @code 
 #include <libssh/callbacks.h>
 ...
-ssh_threads_set_callbacks(ssh_threads_noop);
+ssh_threads_set_callbacks(ssh_threads_get_noop());
 ssh_init();
 @endcode
 
@@ -40,7 +40,7 @@ threading backend:
 @code
 #include <libssh/callbacks.h>
 ...
-ssh_threads_set_callbacks(ssh_threads_pthread);
+ssh_threads_set_callbacks(ssh_threads_get_pthread());
 ssh_init();
 @endcode