update ansible command usage at user document

ansible command need assign hosts to run.

Change-Id: Ifadc48ba06ef1726aa73b21ff37366360cfffbbe

doc/source/user/quickstart.rst

@@ -274,7 +274,7 @@ than one node, edit ``multinode`` inventory:
 
       Ubuntu might not come with python pre-installed. That will cause
       errors in ping module. To quickly install python with ansible you
-      can run ``ansible -m raw -a "apt-get -y install python-dev all"``
+      can run ``ansible -i multinode all -m raw -a "apt-get -y install python-dev"``
 
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