Update instructions for systemd drop-ins

When creating a `/etc/systemd/system/docker.service`
unit file, that file _replaces_ the default
(`/lib/systemd/system/docker.service`) unit file.

While this may be intended by some, this section
describes how to use "partial" unit files ("drop-ins")
to override only certain properties of the default
unit file.

This patch removes the mention of
`/etc/systemd/system/docker.service` to prevent
users from inadvertently replacing the default
unit file.

Signed-off-by: Sebastiaan van Stijn <[email protected]>

engine/admin/systemd.md

@@ -36,9 +36,7 @@ The recommended way is to use a systemd drop-in file (as described in the <a
 target="_blank"
 href="https://www.freedesktop.org/software/systemd/man/systemd.unit.html">systemd.unit</a>
 documentation). These are local files named `<something>.conf` in the
-`/etc/systemd/system/docker.service.d` directory. This could also be
-`/etc/systemd/system/docker.service`, which also works for overriding the
-defaults from `/lib/systemd/system/docker.service`.
+`/etc/systemd/system/docker.service.d` directory.
 
 However, if you had previously used a package which had an `EnvironmentFile`
 (often pointing to `/etc/sysconfig/docker`) then for backwards compatibility,
@@ -160,7 +158,7 @@ you will need to add this configuration in the Docker systemd service file.
 1.  Create a systemd drop-in directory for the docker service:
 
     ```bash
-    $ mkdir /etc/systemd/system/docker.service.d
+    $ mkdir -p /etc/systemd/system/docker.service.d
     ```
 
 2.  Create a file called `/etc/systemd/system/docker.service.d/http-proxy.conf`