doc: Fix systemd spelling and link to doc/init.md

See https://www.freedesktop.org/wiki/Software/systemd/

contrib/init/README.md

@@ -1,6 +1,6 @@
 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.