gh-115114: Add missing slash to file URI prefix file:/

[edgarrmondragon]

• Issue: Typo in "What’s New In Python 3.13", file:/ missing another trailing slash #115114

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[barneygale]

Technically we support URIs like file:c:/foo and file:/c:/foo on Windows, and file:/blah on POSIX, but they're variant formats and not produced by as_uri(). So this change makes sense.