doc: improve formatting of STYLE_GUIDE.md

* Wrap text at 80 characters.
* Use periods consistently.

PR-URL: https://github.com/nodejs/node/pull/13135


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>