project create is missing --parent in doc

'project create' is missing '--parent <project>' argument in doc,
actually it is supported.

Change-Id: Id0edaab4e2b02a1f7d974d71a11c7c373e31806f
Closes-Bug: #1460256

Author: Guojian Shao

doc/source/command-objects/project.rst

@@ -14,6 +14,7 @@ Create new project
 
     os project create
         [--domain <domain>]
+        [--parent <project>]
         [--description <description>]
         [--enable | --disable]
         [--property <key=value>]