fix example CLI flags for the sandbox

[kachick]

> nixpkgs-review --sandbox pr 361460
usage: /nix/store/lzz1i98h78miinzx3yg6ly7kl4ypxd42-nixpkgs-review-3.0.0/bin/nixpkgs-review
       [-h] [--version]
       {post-result,approve,comments,merge,pr,rev,wip} ...
/nix/store/lzz1i98h78miinzx3yg6ly7kl4ypxd42-nixpkgs-review-3.0.0/bin/nixpkgs-review: error: unrecognized arguments: --sandbox

> nixpkgs-review pr --sandbox 361460
-> Attempting to fetch eval results from GitHub actions
-> Successfully fetched rebuilds: no local evaluation needed
...
Using sandbox mode. Some things may break!

[Mic92]

@mergify queue

[mergify]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at eb4f1b1