docs: Make SLES docker guide consistent
Fix wording to make the SUSE SLES docker install guide consistent with
the others.
For more information on installing Docker please refer to the
[Docker Guide](https://www.suse.com/documentation/sles-12/singlehtml/book_sles_docker/book_sles_docker.html).
-2. Configure Docker to use Kata Containers with one of the following methods:
+2. Configure Docker to use Kata Containers by default with one of the following methods:
1. systemd
