Fix exception message in ClusterSettings

Previously, an exception message about srvHost incorrectly referred
to the hosts list.  It's now update to refer to srvHost.

JAVA-4141

Author: jyemin

driver-core/src/main/com/mongodb/connection/ClusterSettings.java

@@ -544,7 +544,7 @@ private ClusterSettings(final Builder builder) {
 
             if (builder.srvHost.split("\\.").length < 3) {
                 throw new IllegalArgumentException(format("An SRV host name '%s' was provided that does not contain at least three parts. "
-                        + "It must contain a hostname, domain name and a top level domain.", builder.hosts.get(0).getHost()));
+                        + "It must contain a hostname, domain name and a top level domain.", builder.srvHost));
             }
         }