tools: remove unneeded parentheses in doc/html.js

PR-URL: https://github.com/nodejs/node/pull/16845
Ref: https://github.com/nodejs/node/pull/16801#discussion_r149142120


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>