Updated c3.xlarge memory size to 7.5GiB

While on the main EC2 [service page](http://aws.amazon.com/ec2/#instance), it states that c3.xlarge instances have 7GiB of memory, on the dedicated instance types [page](http://aws.amazon.com/ec2/instance-types/) it states that it has 7.5GiB. And most importantly: the instance has in fact 7.5GiB of memory.