[docs] [issue25523] Correct "a" article to "an" article

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Thank you Martin for your review.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25523>
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