doc: reduce GitHub template verbosity

Reduce unnecessarily verbose paragraph to a sentence.

PR-URL: https://github.com/nodejs/node/pull/6801


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Myles Borins <myles.borins@gmail.com>