Fix a set checker

[ligurio]

$ java -jar ~/sources/elle-cli/target/elle-cli-0.1.7-standalone.jar --verbose --model set histories/jepsen/set.edn 
{"ok-count":3,
 "valid?":true,
 "lost-count":0,
 "lost":"#{}",
 "acknowledged-count":3,
 "recovered":"#{}",
 "ok":"#{0..2}",
 "attempt-count":3,
 "unexpected":"#{}",
 "unexpected-count":0,
 "recovered-count":0}

Fixes #44