source/2.8/new-2.8.1.txt: fix markup (align with source repo)

Signed-off-by: Jim Klimov <jimklimov@gmail.com>

Author: jimklimov

source/2.8/new-2.8.1.txt

@@ -1,9 +1,5 @@
-ifdef::txt[]
-NUT Release Notes
-=================
-endif::txt[]
-
-If you're upgrading from an earlier version, see the link:UPGRADING.adoc[] file.
+If you're upgrading from an earlier version, see the UPGRADING.adoc file
+in NUT sources.
 
 Please note that web and source document links, product and service names
 listed in historic entries of past releases may no longer be relevant.