[docs] [issue15660] Clarify 0 prefix for width specifier in str.format doc,

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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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/issue15660>
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