editing 16-about-systemd.md

* some sentence simplification
* replacement of passive voice phrases where possible
* since systemd is a daemon (and yes a suite of programs as well) I believe it needs to be consistently entered in lower case, rather than "Systemd" and it needs to be consistently in code back tics too (`systemd`) this **might** be a judgement call on my part. I made an exception for quoted content.
* indented bulleted items need to be 4 spaces, rather than 2
* for the most part (unless complete sentences) bulleted items do not need punctuation or capitalization
* remove some errant spaces at the end of lines
* remove some tab characters within the examples for the unit file
* replaced the "[" and "]" with ++open-bracket++ and ++close-bracket++ respectively
* Where System V and SystemV are noted, replaced with the more commonly accepted SysV for uniformity
* ensure fenced code blocks have titles (example ```bash or ```text)

Author: sspencerwire