Fix an inaccurate comment (#4315)

Summary:
According to 4848bd0c4e/db/log_reader.cc (L355), the original text is misleading when describing the layout of RecyclableLogHeader.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4315

Differential Revision: D9505284

Pulled By: riversand963

fbshipit-source-id: 79994c37a69e7003f03453e7efc0186feeafa609

db/log_format.h

@@ -37,9 +37,9 @@ static const unsigned int kBlockSize = 32768;
 // Header is checksum (4 bytes), length (2 bytes), type (1 byte)
 static const int kHeaderSize = 4 + 2 + 1;
 
-// Recyclable header is checksum (4 bytes), type (1 byte), log number
-// (4 bytes), length (2 bytes).
-static const int kRecyclableHeaderSize = 4 + 1 + 4 + 2;
+// Recyclable header is checksum (4 bytes), length (2 bytes), type (1 byte),
+// log number (4 bytes).
+static const int kRecyclableHeaderSize = 4 + 2 + 1 + 4;
 
 }  // namespace log
 }  // namespace rocksdb