fix str.format() call in modules.boto_elb.deregister_instances

************* Module salt.modules.boto_elb
salt/modules/boto_elb.py:585: [E1306(too-few-format-args), deregister_instances] Not enough arguments for format string
Merge pull request #15554 from hulu/boto_elb

fix str.format() call in modules.boto_elb.deregister_instances