docs: networking: Fix bridge documentation URL

Current documentation URL [1] is no longer valid.

[1] https://www.linuxfoundation.org/collaborate/workgroups/networking/bridge

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230124145127.189221-1-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>