twitter-server: Fixed spelling
twitter-server: Fixed spelling

Summary: Logback was spelled incorrectly. This change addresses that.

Logbcak to Logback

Signed-off-by: Yufan Gong <yufang@twitter.com>

Differential Revision: https://phabricator.twitter.biz/D120078

twitter/twitter-server,twitter/twitter-server,twitter/twitter-server