router: docs: add horizontal line before nested command docs

Lines before commanmd groups are missing (i.e. see proxmox-backup-client command
syntax docs)

Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>

proxmox-router/src/cli/format.rs

@@ -415,6 +415,9 @@ fn generate_nested_usage_do<'cli>(
                 ));
             }
             CommandLineInterface::Nested(map) => {
+                if format == DocumentationFormat::ReST {
+                    usage.push_str("\n----\n\n");
+                }
                 usage.push_str(&generate_nested_usage_do(state, &new_prefix, map, format));
             }
         }