Added missing backslash to documentation sites cli snippet

I think the cli code block misses a backslash to brevent line break when copy/pasting it to a terminal.
I doubt that this is intentional, if it is, feel free to reject the pr.

Signed-off-by: Julian <gitea+julian@ic.thejulian.uk>
(cherry picked from commit 895e7a3df8)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/service_create.md

@@ -291,7 +291,7 @@ service with two labels:
 ```console
 $ docker service create \
   --name redis_2 \
-  --label com.example.foo="bar"
+  --label com.example.foo="bar" \
   --label bar=baz \
   redis:3.0.6
 ```