tools: replace string concatenation with template literals

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

PR-URL: https://github.com/nodejs/node/pull/16804


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>