doc: fix style bug on version link dropdown

Created by: chris--young

I noticed a minor styling bug while working on https://github.com/nodejs/node/pull/15670 that this pr will fix.

Before:

After:

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc