Fix broken link to Relay (#322)

* Fix broken link to Relay

Fixed the link to the Relay network layer page. Also renamed Relay Modern to just Relay to match now Relay now generally refers to its current version.

* Used updated link

Author: breyed

src/pages/postgraphile/security.md

@@ -155,7 +155,7 @@ const client = new ApolloClient({
 ```
 
 or
-[with Relay Modern](https://facebook.github.io/relay/docs/en/network-layer.html):
+[with Relay](https://relay.dev/docs/guides/network-layer/)
 
 ```js{3,8}
 function fetchQuery( operation, variables, cacheConfig, uploadables) {