Fixed missing task name in bundle command to create realm

When I run the command from README.md I get this error: 

vagrant@precise64:/vagrant/src/checkpoint$ bundle exec ./bin/checkpoint create example -t "Example Security Realm" -d 192.168.33.10
WARNING: Nokogiri was built against LibXML version 2.8.0, but has dynamically loaded 2.7.8
Could not find task "create".

When I add "realm" it seems to work. This pull request is about fixing the documentation.