Fix div not allowed in summary element in ToC

[danielfdickinson]

What does this PR change? What problem does it solve?

As per Mozilla Developer Network detail summary element page only 'phrasing elements' and/or one element of heading content are allowed inside a <summary> element.

Once again found by (valid) html validator complaint (I turn off the parts that are about opinions vs. HTML standards).

The table of contents <summary> currently contains a <div>.

I have replaced the <div> with a <span> and it appears to work on https://www.princesandmadmen.ca/post/schizophrenia-talk/ although increasing the font-size might be desirable.

Was the change discussed in an issue or in the Discussions before?

Closes #550

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.

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