Backport commit a95062e03 from Emacs

* doc/org-manual.org (Summary): Remove broken link to Network Theory Ltd.

Delete broken link to Network Theory Ltd.
a95062e036d9d5fc45d9a3ba1c10b0bd9b587399
Stefan Kangas
Sat Oct 15 13:06:45 2022 +0200

[ km: Note that this is already covered by Org's 58a46fab0 and that
  version will be taken in the merge. ]

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 published as a paperback book by
+Network Theory Ltd. in 2010.
 
 ** Installation
 :PROPERTIES: