[issue38351] Modernize email example from %-formatting to f-string

[Mariatta]

Mariatta <mariatta at python.org> added the comment:

I think updating one isolated code example is less invasive and easier to review, instead of one big PR to change everything from "%s" to str.format or f-string.

I brought up this example code merely because I happen to be reading this page.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38351>
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