Fixed docopt instance creation

Invalid key for first parameter of docopt(). Change
the code to use a standard parameter for the first
parameter of the docopt() object

kiwi_boxed_plugin/tasks/system_boxbuild.py

@@ -215,7 +215,7 @@ def _validate_kiwi_build_command(self) -> List[str]:
             )
         )
         validated_build_command = docopt(
-            doc=kiwi.tasks.system_build.__doc__,
+            kiwi.tasks.system_build.__doc__,
             argv=kiwi_build_command
         )
         # rebuild kiwi build command from validated docopt parser result