Fix example of in-buffer setting of archive location in manual

* doc/org-manual.org (Summary of In-Buffer Settings): Fix archive
example.

The provided example '%s_done' is not valid syntax for archive
location. Should be '%s_done::'.

doc/org-manual.org

@@ -18854,7 +18854,7 @@ Closing and reopening the Org file in Emacs also activates the
 changes.
 
 #+attr_texinfo: :sep ,
-- =#+ARCHIVE: %s_done= ::
+- =#+ARCHIVE: %s_done::= ::
 
   #+cindex: @samp{ARCHIVE}, keyword
   #+vindex: org-archive-location