Minor Description
Hi, I find the description "如下所示，二阶差分不支持布尔类型的编码" is not correct. Because TS_2DIFF(二阶差分) isn't a type of encoding.
the description is at http://iotdb.apache.org/zh/UserGuide/Master/Concept/Data%20Type.html#%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B