Update WMS bbox placeholder test url

[merydian]

Description

Make the test URL in TestQgsWmsProvider::testBboxPlaceholderReplacement() be less confusing.

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[github-actions]

🪟 Windows Qt6 builds

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(Built from commit 88c9b41)

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This app is not notarized, run sudo xattr -d com.apple.quarantine /Applications/QGIS*.app to avoid the warning
(Built from commit 88c9b41)

[github-actions]

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