manual: Fix installation instructions

* doc/org-manual.org (Using Emacs packaging system): Fix installation
instructions.  `M-x package-install RET org' does not work because
"org" is a built-in package, so it is not available for installation.

doc/org-manual.org

@@ -97,8 +97,9 @@ We *strongly recommend* sticking to a single installation method.
 :END:
 
 Recent Emacs distributions include a packaging system which lets you
-install Elisp libraries.  You can install Org with {{{kbd(M-x
-package-install RET org)}}}.
+install Elisp libraries.  You can install Org from the "package menu",
+with {{{kbd(M-x list-packages)}}}.  See [[info:emacs::Package Menu][Package Menu]] for more
+information.
 
 #+attr_texinfo: :tag Important
 #+begin_quote