Add missing parenthesis to email heading recent text

Add missing parenthesis to email heading recent text

Fix https://gitlab.com/gitlab-org/gitter/webapp/-/issues/2549

Testing strategy

1. node build-scripts/mandrill-templates/test.js
2. Copy the contents of build-scripts/mandrill-templates/unread-notification.out.html (or preview directly in a browser)

If you want to send it to one of you email clients to see how it would look:

1. Paste the contents into Mailchimps CSS Inliner Tool
2. Create a script using nodemailer to send the converted HTML to a variety of email addresses to test in a variety of email clients on various devices (inc webmail) and using various email services (eg. Yahoo! Mail, Gmail, Outlook Live/Hotmail, iCloud and plain old POP3/IMAP)