doc/org-manual.org: Mention that Network Theory Ltd. is closed

See https://list.orgmode.org/orgmode/87edv9ti73.fsf@no.workgroup/

Suggested-by: Gregor Zattler <telegraph@gmx.net>

doc/org-manual.org

@@ -68,8 +68,8 @@ of Org, as well as additional information, frequently asked questions
 [[https://orgmode.org]].
 
 #+cindex: print edition
-An earlier version (7.3) of this manual is available as a [[http://www.network-theory.co.uk/org/manual/][paperback
-book from Network Theory Ltd.]].
+An earlier version (7.3) of this manual was available as a paperback
+book from the Network Theory Ltd. publishing company, closed in 2009.
 
 ** Installation
 :PROPERTIES: