Corrected max-file option - was incorrectly spelt as max-files

Signed-off-by: Steve Richards <steve.richards@docker.com>
(cherry picked from commit 04f88005c9)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/dockerd.md

@@ -1312,7 +1312,7 @@ This is a full example of the allowed configuration options on Linux:
 	"log-driver": "json-file",
 	"log-opts": {
 		"max-size": "10m",
-		"max-files":"5",
+		"max-file":"5",
 		"labels": "somelabel",
 		"env": "os,customer"
 	},