guide: Match recommended global keybindings to manual's

* doc/org-guide.org (Activation): Match recommended global keybindings
to those in the manual.

In the guide's .texi to .org conversion in 1a678f184 (Generate compact
guide from an Org file, 2019-03-19), the leading "C-c"s of the
bindings were lost.  Restore them.

Also, drop org-switchb, which was pruned from the manual's suggested
bindings in d03515590, and tweak the surrounding text more closely
match the manual's.

Reported-by: Nick Dokos <ndokos@gmail.com>

doc/org-guide.org

@@ -72,16 +72,14 @@ the following command to generate autoload information.
 :UNNUMBERED: notoc
 :END:
 
-Add the following lines to your Emacs init file.  The last four lines
-define /global/ keys for some commands---please choose suitable keys
-yourself.
+Add the following lines to your Emacs init file to define /global/
+keys for three commands that are useful in any Emacs buffer, not just
+Org buffers.  Please choose suitable keys yourself.
 
 #+begin_src emacs-lisp
-;; The following lines are always needed.  Choose your own keys.
-(global-set-key (kbd "C-l") 'org-store-link)
-(global-set-key (kbd "C-a") 'org-agenda)
-(global-set-key (kbd "C-c") 'org-capture)
-(global-set-key (kbd "C-b") 'org-switchb)
+(global-set-key (kbd "C-c l") 'org-store-link)
+(global-set-key (kbd "C-c a") 'org-agenda)
+(global-set-key (kbd "C-c c") 'org-capture)
 #+end_src
 
 Files with extension =.org= will be put into Org mode automatically.