Fix obsolete parameter name in HiveConf description of hive.hashtable.initialCapacity

The description of hive.hashtable.initialCapacity in HiveConf.java refers to a parameter that existed in an early patch for HIVE-7616 ("hive.hashtable.stats.key.estimate.adjustment") but was renamed in later patches. So change hive.hashtable.stats.key.estimate.adjustment to hive.hashtable.key.count.adjustment in this parameter definition in HiveConf.java:

    HIVEHASHTABLETHRESHOLD("hive.hashtable.initialCapacity", 100000, "Initial capacity of " +
        "mapjoin hashtable if statistics are absent, or if hive.hashtable.stats.key.estimate.adjustment is set to 0"),