Fixes some nits and blog post link in upgrade-to/v6.mdx

[ArmandPhilippot]

Description (required)

I forgot I had this in my Git stash... A few things I spotted while translating the v6 upgrade guide:

• we use the beta blog post instead of the v6 one
• some typo or British spelling around "behavior"
• some minor nits for consistency (parentheses for functions, missing dot...)

Related issues & labels (optional)

• Suggested label: consistency/formatting, typo/link/grammar - quick fix!

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en/guides/upgrade-to/v6.mdx	Source changed, localizations will be marked as outdated.

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.