src: add newline in --help message

In the current node.js 8.0.0 release, there is a missing newline in the --help command line flag message. I've added a newline where appropriate. Screenshots attached showing the change. before: after:

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows [commit guidelines]

Affected core subsystem(s)

src