Merge pull request NixOS#14549 from Alexis211/doc/fix-nar-format

doc: fix "Nix Archive (NAR) format" specification

Author: Ericson2314

doc/manual/source/protocols/nix-archive/index.md

@@ -24,7 +24,7 @@ nar-obj-inner
   | str("type"), str("directory") directory
   ;
 
-regular = [ str("executable") ], str("contents"), str(contents);
+regular = [ str("executable"), str("") ], str("contents"), str(contents);
 
 symlink = str("target"), str(target);
 
@@ -52,4 +52,4 @@ The Nix Archive (NAR) format is also formally described using [Kaitai Struct](ht
 {{#include nar.ksy}}
 ```
 
-The source of the spec can be found [here](https://github.com/nixos/nix/blob/master/src/nix-manual/source/protocols/nix-archive/nar.ksy). Contributions and improvements to the spec are welcomed.
+The source of the spec can be found [here](https://github.com/nixos/nix/blob/master/src/nix-manual/source/protocols/nix-archive/nar.ksy). Contributions and improvements to the spec are welcomed.