[docs] [issue18981] Typo in the ctypes tests

[Anoop Thomas Mathew]

Changes by Anoop Thomas Mathew <atmb4u at gmail.com>:


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nosy: +amaury.forgeotdarc, belopolsky, meador.inge

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18981>
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