tools: replace string concatenation with template literals

Replace a string concatenation in the processList function
in doc/json.js.

If missing, initialize the textRow property in the same line of the
concatenation.

PR-URL: https://github.com/nodejs/node/pull/16806


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