Correct user DB format docs re REML v6

Description of REML format that may be assumed by readers of DB format
v2 was change to from REML v5 to v6.

Author: delphidabbler

Docs/Design/FileFormats/user-db.html

@@ -1100,7 +1100,7 @@ <h3>
 </ul>
 
 <p>
-  Readers of v2 and later files may parse REML from any file version as if it were REML v5, since all versions of REML up to v5 are compatible.
+  Readers of v2 and later files may parse REML from any file version as if it were REML v6, since all versions of REML up to v6 are compatible.
 </p>
 
 <h3>