Fix type error of EC2Tests.NetworkInterfaceAttributeTests

Fix following compilation error.

```
test/EC2Tests/NetworkInterfaceAttributeTests.hs:78:74:
    Couldn't match type `Maybe Text' with `Text'
    Expected type: Group -> Text
      Actual type: Group -> Maybe Text
    In the first argument of `map', namely `groupId'
    In the second argument of `modifyNetworkInterfaceSecurityGroup', namely
      `(map groupId sgs)'
    In a stmt of a 'do' block:
      modifyNetworkInterfaceSecurityGroup nic (map groupId sgs)
```