org-manual.org: Fix repository link

* doc/org-manual.org (Using Org's git repository): Fix
repository link.

Thanks to Gustavo Barros for reporting this.

doc/org-manual.org

@@ -142,7 +142,7 @@ You can clone Org's repository and install Org like this:
 
 #+begin_example
 $ cd ~/src/
-$ git clone git@code.orgmode.org:bzg/org-mode.git
+$ git clone https://code.orgmode.org/bzg/org-mode.git
 $ cd org-mode/
 $ make autoloads
 #+end_example