Removed unnecessary comma in billing documentation

[Paulitos]

Why: Remove unnecessary comma in adding-a-sales-tax-certificate.md

Closes: Issue #34672

What's being changed (if available, include any code snippets, screenshots, or gifs):

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
billing/managing-your-github-billing-settings/adding-a-sales-tax-certificate.md	fpt ghec ghes@ 3.14 3.13	fpt ghec ghes@ 3.14 3.13

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[subatoi]

Thank you! I'll get this merged.

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