fix doc typos for setting properties

In file basic_config/gemfire_properties/setting_distributed_properties.html, this text is wrong:

System.setProperty("DgemfirePropertyFile", "gfTest");
System.setProperty("Dgemfire.mcast-port", "10999");

The capital D at the beginning of each property name should be deleted.

In the same file, the reference to DistributedSystem javadocs should be changed to refer to this class:
org.apache.geode.distributed.ConfigurationProperties

ConfigurationProperties also defines constants for each property name so that applications can use them instead of embedding string constants in their code. This has the benefit of avoiding typing mistakes.