Remove irrelevant reference to CIDR ranges.

describe-db-security-groups help makes reference to external documentation
re: CIDR ranges.  This seems out of context for a function which simply
returns data.

![screen shot 2013-07-01 at 14 41 41](https://f.cloud.github.com/assets/1309226/730444/01d4c8aa-e254-11e2-9c12-c1bbd785dd70.png)