Remove superfluous ? from sitemap.xml rewrite rule

[szepeviktor]

Fix a bug: there were two question marks in the URL.

Trac ticket: https://core.trac.wordpress.org/ticket/63699

[github-actions]

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Props szepeviktor.

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[szepeviktor]

Not again! @swissspidy is the destroyer of the Internet.
f9bd807

[github-actions]

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60458
GitHub commit: 8671dfe

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.

[szepeviktor]

@swissspidy Thank you.