Fixed typo error

Parameter 'groupName' doesn't live inside 'monitoring' section of XML response.

Also I'm not sure that this a valid parameter inside an 'instancesSet' tag.
I've looked documentation for EC2 API version 2010-08-31 and cannot find this parameter as a direct child of instancesSet/item element.

Furthermore, in current version of EC2 API there are a lot of 'groupName' tags and it's is possible that this parameter takes an ambiguous value.