[docs] Tutorial 2.1.1: name 'sys' is not defined

[Krzysztof Żelechowski]

Regarding the section "Argument Passing" [1]:

== Problem statement ==
When the student tells python to evaluate sys.argv[0] as described, she gets 
the following error: 

name 'sys' is not defined

== Suggestion ==
Please append the following example:

import sys
sys.argv[0]

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[1] <URL:http://docs.python.org/tutorial/interpreter.html#argument-passing>