docs: update references to s6-rc (now using systemd)

README.md

@@ -102,7 +102,7 @@ i.e. you might find value in using these in your own config:
 - `modules/users/`
   - convenience layer atop the above modules so that you can just write
     `fs.".config/git"` instead of `fs."/home/colin/.config/git"`
-  - per-user services managed by [s6-rc](https://www.skarnet.org/software/s6-rc/)
+  - simplified `systemd.services` API
 
 some things in here could easily find broader use. if you would find benefit in
 them being factored out of my config, message me and we could work to make that happen.

hosts/common/programs/mmcli.nix

@@ -1,7 +1,7 @@
 # GPS:
 # - enable: `mmcli --modem any --location-enable-gps-unmanaged`
 #   - or `mmcli -m any --location-enable-gps-nmea`
-#   - or use `s6-rc start eg25-control-gps`
+#   - or use `systemctl start eg25-control-gps`
 # - verify GPS is enabled: `mmcli --modem any --location-status`
 # - query GPS coordinates: `mmcli -m any --location-get`
 # - monitor constellation info: `mmcli -m any --location-monitor`