Update README.md

Specifying security group ID gives the following error without starting any EC2
The security group 'sg-xxxxxxxx' does not exist in default VPC 'vpc-xxxxxxxx' (Fog::Compute::AWS::NotFound)
The EC2 starts after specifying the relevant Security group Name for that ID.
Merge pull request #231 from droidlabour/patch-1

Update README.md