json_format.md uses negative errno

The [`errno`](https://github.com/rust-vmm/seccompiler/blob/main/docs/json_format.md) value in the `mismatch_action` needs to be positive:

```
JsonFrontend(SerdeJson(Error("invalid value: integer `-1`, expected u32", line: 5, column: 0)))
```