fix README for insecure registries

option is called insecure-registries

Change-Id: I125be2f8cb2a603e93896a387660b1b5fc20057c
diff --git a/README.rst b/README.rst
index b772a60..ff95bc1 100644
--- a/README.rst
+++ b/README.rst
@@ -18,7 +18,7 @@
         enabled: true
         options:
           bip: 172.31.255.1/16
-          insecure_registries:
+          insecure-registries:
             - 127.0.0.1
             - 10.0.0.1
           log-driver: json-file