org.texi: Fix protocol for read-only repository

* doc/org.texi (Installation): Fix protocol for read-only repository.

doc/org.texi

@@ -921,7 +921,7 @@ You can clone Org's repository and install Org like this:
 
 @example
 $ cd ~/src/
-$ git clone git@@code.orgmode.org:bzg/org-mode.git
+$ git clone https://code.orgmode.org/bzg/org-mode.git
 $ make autoloads
 @end example