Fix typo in error message
Fix typo in error message

Closes https://bugs.launchpad.net/graphite/+bug/1007893

cbowman0/whisper,graphite-server/whisper,alexandreboisvert/whisper,kerlandsson/whisper,deniszh/whisper,akbooer/whisper,acdha/whisper,jjneely/whisper,piotr1212/whisper,penpen/whisper,obfuscurity/whisper,graphite-project/whisper