Fix incorrect LSN format in comment.

The comment previously used %X/08X, which is wrong.
Updated it to the standardized format %X/%08X.

Author: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/ME0P300MB0445A37908EFCCD15E6D749DB62BA@ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM

src/include/access/xlogdefs.h

@@ -38,7 +38,7 @@ typedef uint64 XLogRecPtr;
 /*
  * Handy macro for printing XLogRecPtr in conventional format, e.g.,
  *
- * printf("%X/08X", LSN_FORMAT_ARGS(lsn));
+ * printf("%X/%08X", LSN_FORMAT_ARGS(lsn));
  *
  * To avoid breaking translatable messages, we're directly applying the
  * LSN format instead of using a macro.