fix review code

Author: Zheaoli

Lib/tarfile.py

@@ -480,7 +480,7 @@ def _init_read_gz(self):
         # taken from gzip.GzipFile with some alterations
         if self.__read(2) != b"\037\213":
             raise ReadError("not a gzip file")
-        method, flag = struct.unpack("<BB4x", self.__read(8))
+        method, flag = struct.unpack("<BB6x", self.__read(8))
         if method != 8:
             raise CompressionError("unsupported compression method")