Fix json format of plugin

Signed-off-by: Janonymous <janonymous.codevulture@gmail.com>
(cherry picked from commit 2d24dbe896b33ab2b450ace457b0e266b27dfd74)
Signed-off-by: Victor Vieux <vieux@docker.com>

docs/extend/plugin_api.md

@@ -79,7 +79,7 @@ This is the JSON format for a plugin:
     "InsecureSkipVerify": false,
     "CAFile": "/usr/shared/docker/certs/example-ca.pem",
     "CertFile": "/usr/shared/docker/certs/example-cert.pem",
-    "KeyFile": "/usr/shared/docker/certs/example-key.pem",
+    "KeyFile": "/usr/shared/docker/certs/example-key.pem"
   }
 }
 ```