Fix cover generation

[svillar]

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

Generating cover images was failing with an error in "partials/cover.html" at line 4, with the message at <$.IsSingle>: invalid value; expected bool

I am not fluent in Go at all but threating IsSingle as a string seems to fix the problem

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

#1417

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• [x ] 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.
• [ x] This change does not include any CDN resources/links.
• [ x] 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.