Fix LXD setup in snap README (#8416)
Fix LXD setup in snap README (#8416)

Fixes #8409.

Change the line in the README to allow `sudo /snap/bin/lxd.migrate -yes` to fail (for example, if there's nothing to migrate), but the whole command to succeed.

I tested this on a clean Focal install and confirmed it works.
stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,lmcro/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt