tools: replace string concatenation with template

Replace string concatenation in tools/doc/type-parser.js
with template literals

PR-URL: https://github.com/nodejs/node/pull/15827


Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>