Fix errored example in README.rst

The given example for authorize-security-group-ingress --ip-permissions uses cidr_ip which is incorrect:

$ aws ec2 authorize-security-group-ingress --group-name MySecurityGroup   --ip-permissions '{"FromPort":22,"ToPort":22,"IpProtocol":"tcp","IpRanges":[{"cidr_ip": "192.168.0.0/24"}]}'
Invalid value ('cidr_ip') for param element of list:IpPermissions of type list 

It should be CidrIp