Update security vulnerability reporting method and mention bug bounty program (#1627)
Update security vulnerability reporting method and mention bug bounty program (#1627)

Now that HackerOne is used, update security reporting instructions
to refer to https://hackerone.com/nodejs. Additionally, mention the new bug
bounty program.

Also, correct a few other minor issues with the page.
yous/new.nodejs.org,phillipj/new.nodejs.org,nodejs/new.nodejs.org,nodejs/new.nodejs.org,yous/new.nodejs.org,phillipj/new.nodejs.org