Fix markup in Zen discovery docs

This commit fixes a markup issue in the Zen discovery docs where a link
and its referring text were not on the same line tripping the renderer.

docs/reference/modules/discovery/zen.asciidoc

@@ -30,9 +30,8 @@ http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`netwo
 http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java security policy]. Any hosts that
 fail to resolve will be logged. Note also that with the Java security manager in place, the JVM defaults to caching
 negative hostname resolutions for ten seconds. This can be modified by adding
-http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html
-[`networkaddress.cache.negative.ttl=<timeout>`] to your
- http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java security policy].
+http://docs.oracle.com/javase/8/docs/technotes/guides/net/properties.html[`networkaddress.cache.negative.ttl=<timeout>`]
+to your http://docs.oracle.com/javase/8/docs/technotes/guides/security/PolicyFiles.html[Java security policy].
 
 Unicast discovery provides the following settings with the `discovery.zen.ping.unicast` prefix: