doc: run oldconfig on xtools config

This is needed to reset the configuration options based on the OS we are
building on.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

doc/getting_started/installation_mac.rst

diff --git a/doc/getting_started/installation_mac.rst b/doc/getting_started/installation_mac.rst
index c327ea1..8044f63 100644
--- a/doc/getting_started/installation_mac.rst
+++ b/doc/getting_started/installation_mac.rst
@@ -152,7 +152,7 @@
 .. code-block:: console
 
    $ export CT_PREFIX=/Volumes/CrossToolNG
-   $ ct-ng menuconfig
+   $ ct-ng oldconfig
 
 Verifying the Configuration of the Toolchain
 ============================================