Update .env.example WIREGUARD_PEERS -> PEERS

[pawl]

Fixes #111 and #113

Currently docker-wireguard uses the PEERS environment variable and not the WIREGUARD_PEERS that's currently in wirehole's docker-compose file. Example: Link to docker-wireguard search for PEERS

This results in new users of wirehole not seeing any QR codes or wireguard configs generated on their first start.

More details: https://github.com/IAmStoxe/wirehole/issues/111#issuecomment-1837201951

This PR fixes the issue by updating WIREGUARD_PEERS to PEERS to match docker-wireguard.

[raspberrycoulis]

Just found this issue and the fix worked for me. First time I've used this and it failed without the fix shown here.

[MxD-js]

Yep, this also fixed it for me. Thanks.