fix: disqus.html (#1876)
fix: disqus.html (#1876)

Using 'let' keyword doesn't properly work for the disqus configuration variables. 
The original Hugo disqus template uses 'var' (https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/disqus.html),
and the Disqus documentation (https://help.disqus.com/en/articles/1717084-javascript-configuration-variables) also uses the 'var' keyword.
gcushen/hugo-academic,gcushen/hugo-academic,gcushen/hugo-academic,gcushen/hugo-academic