Fix the usage info in cf feature-flag command

Before Fix: If cf user does
'cf feature-flag'
the usage is
USAGE:
   cf feature-flag

After Fix:If cf user does
'cf feature-flag'
the usage is
USAGE:
   cf feature-flag FEATURE_NAME

http://rnd-github.huawei.com/paas/cli/issues/21
Merge pull request #13 from SrinivasChilveri/hw-issue-21

Fix the usage info in cf feature-flag command