message: doc: Fix systemd spelling and link to doc/init.md
  See
diff: Sample configuration files for:
  ```
  -SystemD: bitcoind.service
  +systemd: bitcoind.service
  Upstart: bitcoind.conf
  OpenRC: bitcoind.openrc
  bitcoind.openrcconf
  @@ -9,4 +9,4 @@ macOS: org.bitcoin.bitcoind.plist
  ```
  have been made available to assist packagers in creating node packages here.
  -See doc/init.md for more information.
  +See [doc/init.md](../../doc/init.md) for more information.
  