trivial: tweak sentence in ubuntu install step

Author: kay-kim

source/includes/steps-install-mongodb-on-ubuntu.yaml

@@ -19,17 +19,18 @@ level: 4
 ref: sources-list
 content: |
 
+   Create the list file for your version of Ubuntu.
+
    .. tabs::
 
       tabs:
          - id: trusty
            name: Ubuntu 14.04 (Trusty)
            content: |
 
-              Create the
+              Use the following command to create the
               ``/etc/apt/sources.list.d/mongodb-org-{+version+}.list``
-              list file using the command appropriate for your version
-              of Ubuntu:
+              file:
 
               .. code-block:: sh