Fix broken link: Update cost-management-+-billing.md

[learn-build-service-prod[bot]]

Learn Build status updates of commit ba5340d:

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docs-ref-services/legacy/cost-management-+-billing.md	:white_check_mark:Succeeded	View (azure-python-previous)

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[learn-build-service-prod[bot]]

Learn Build status updates of commit 0bd71d9:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs-ref-services/legacy/cost-management-+-billing.md	:white_check_mark:Succeeded	View (azure-python-previous)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[JasonWHowell]

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[JasonWHowell]

@danieljurek can you review PR #1865 - just fixing a broken link.