doc: Use more common doc for gdb

Signed-off-by: Petr Vorel <pvorel@suse.cz>

doc/developers/debugging.rst

@@ -15,7 +15,8 @@ Tracing and debugging syscalls
 ------------------------------
 
 The new test library runs the actual test (i.e. the ``test()`` function) in a
-forked process. To get stack trace of a crashing test in gdb it's needed to
-`set follow-fork-mode child <https://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_25.html>`_.
+forked process. To get stack trace of a crashing test in ``gdb`` it's needed to
+`set follow-fork-mode child <https://sourceware.org/gdb/current/onlinedocs/gdb.html/Forks.html>`_.
+
 To trace the test, please use ``strace -f`` to enable tracing also for the
 forked processes.