Fix the missing os in command in example

This patch correct the missing one 'os example'.

Change-Id: I7c2cb01082f1eff1a7a607508f166dd54a450f44

Author: ZhaoBo

doc/source/command-beta.rst

@@ -45,7 +45,7 @@ List examples
 .. program:: example list
 .. code:: bash
 
-    os example list
+    openstack example list
 
 Help
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