Docs: Quoted "classes" option key (fixes #6440)

[originalfoo]

Made classes key consistent with all other option keys which are quoted..

[eslintbot]

Thanks for the pull request, @aubergine10! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.
• Pull requests with code require an issue to be mentioned at the end of the commit summary, such as (fixes #1234). Please update the commit summary with an issue (file a new issue if one doesn't already exist).

Can you please update the pull request to address these?

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[mention-bot]

By analyzing the blame information on this pull request, we identified @alberto, @scriptdaemon and @lo1tuma to be potential reviewers

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[eslintbot]

Thanks for the pull request, @aubergine10! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.
• Pull requests with code require an issue to be mentioned at the end of the commit summary, such as (fixes #1234). Please update the commit summary with an issue (file a new issue if one doesn't already exist).

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[originalfoo]

Original ticket #6440

[ilyavolodin]

Looks good. Thanks!