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
Merge branch 'hw-issue-21' of github.com:HuaweiTech/cli into HuaweiTech-hw-issue-21
Merge branch 'HuaweiTech-hw-issue-21'