Fix warning in kaitai spec

Warning:
```
[39/483] Generating src/kaitai-struct-checks/kaitai-generated-sources with a custom command
../src/kaitai-struct-checks/nar.ksy: /types/padded_str/seq/1/encoding:
        warning: use canonical encoding name `ASCII` instead of `ascii` (see https://doc.kaitai.io/ksy_style_guide.html#encoding-name)
```

Author: fzakaria

doc/manual/source/protocols/nix-archive/nar.ksy

@@ -29,7 +29,7 @@ types:
       - id: body
         type: str
         size: len_str
-        encoding: 'ascii'
+        encoding: 'ASCII'
       - id: padding
         size: (8 - (len_str % 8)) % 8