Fix the Usage info in cf security-groups command

Before Fix: If cf user does
'cf security-groups blahblah'
the usage is
USAGE:
   cf security-group

After Fix:If cf user does
'cf security-groups blahblah'
the usage is
USAGE:
   cf security-groups

http://rnd-github.huawei.com/paas/cli/issues/20
Merge pull request #12 from SrinivasChilveri/hw-issue-20

Fix the Usage info in cf security-groups command