org-manual: Fix clone instructions

The same fix was applied to org.texi in b1a6395df.

doc/org-manual.org

@@ -136,6 +136,7 @@ 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
+$ cd org-mode/
 $ make autoloads
 #+end_example
 

doc/org.texi

@@ -621,6 +621,7 @@ You can clone Org's repository and install Org like this:
 @example
 $ cd ~/src/
 $ git clone git@@code.orgmode.org:bzg/org-mode.git
+$ cd org-mode/
 $ make autoloads
 @end example