Fix HTML output example in upgrade guide

[hfournier]

Description (required)

Update HTML output example for heading id in v6 upgrade guide, added missing closing tag: </code>.

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en/guides/upgrade-to/v6.mdx	Localization changed, will be marked as complete.

Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[ArmandPhilippot]

Nice catch, thanks!