tools: replace string concetation with templates

Replace string concatenation in tools/doc/html.js with template
literals.

PR-URL: https://github.com/nodejs/node/pull/16801


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>