RBF: Fix java doc for StateStoreFileSystemImpl

/**
 *StateStoreDriver}implementation based on a filesystem. The most common uses
 * HDFS as a backend.
 */

/**
 * {@link StateStoreDriver} implementation based on a filesystem. The common
 * implementation uses HDFS as a backend. The path can be specified setting
 * dfs.federation.router.driver.fs.path=hdfs://host:port/path/to/store.
 */