doc: fix per-service cgroup syntax

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Author: troglobit

doc/config/service-opts.md

@@ -43,7 +43,7 @@ started at any time by running `initctl start <service>`.
 Other run/task/service options are:
 
   * `caps:...` -- see the [Linux Capabilities](capabilities.md) section
-  * `cgroups:...` -- see the [Cgroups](cgroups.md) section
+  * `cgroup.NAME[,opts]` or `cgroup:opts` -- see the [Cgroups](cgroups.md) section
   * `env:[-]/path/to/env` -- see the [Service Environment](service-env.md) section
   * `log:...` -- see [Redirecting Output](logging.md#redirecting-output)
   * `nowarn` -- see [Conditional Loading](services.md#conditional-loading)