Fix systemd override example in configuring docs

When overriding a systemd configuration via a drop-in file, the
[Service] header is required. This commit adds this to an example
drop-in override in the configuring docs.

Relates #22038

docs/reference/setup/sysconfig/configuring.asciidoc

@@ -98,6 +98,7 @@ any changes in that file, such as:
 
 [source,sh]
 ---------------------------------
+[Service]
 LimitMEMLOCK=infinity
 ---------------------------------