registry: remove protocol from insecure-registries config

Using a protocol here is meaningless, and is ignored by the daemon (and
will even produce a warning in the logs).

Signed-off-by: Justin Chadwell <me@jedevc.com>

registry/insecure.md

@@ -33,7 +33,7 @@ isolated testing or in a tightly controlled, air-gapped environment.
 
     ```json
     {
-      "insecure-registries" : ["http://myregistrydomain.com:5000"]
+      "insecure-registries" : ["myregistrydomain.com:5000"]
     }
     ```