Fix the name of the package that provides apt-add-repository on Ubuntu

docsite/rst/intro_installation.rst

@@ -177,11 +177,13 @@ To configure the PPA on your machine and install ansible run these commands:
 
 .. code-block:: bash
 
-    $ sudo apt-get install apt-add-repository
+    $ sudo apt-get install software-properties-common
     $ sudo apt-add-repository ppa:rquillo/ansible
     $ sudo apt-get update
     $ sudo apt-get install ansible
 
+.. note:: On older Ubuntu distributions, "software-properties-common" is called "python-software-properties".
+
 Debian/Ubuntu packages can also be built from the source checkout, run:
 
 .. code-block:: bash