Update index.rst

I was following this guide.
To install ros2-navigation, need to edit `ros2-dashing-navigation2` to `ros-dashing-navigation2`.

When I tried original command from this guide, I got error below.

```bash
$ sudo apt install ros2-dashing-navigation2
Reading package lists... Done
Building dependency tree... 50%
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros2-dashing-navigation2
$ sudo apt install ros2-dashing-nav2-bringup
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros2-dashing-nav2-bringup
```

sphinx_doc/getting_started/index.rst

@@ -29,8 +29,8 @@ Installation
 
    .. code-block:: bash
 
-      sudo apt install ros2-dashing-navigation2
-      sudo apt install ros2-dashing-nav2-bringup
+      sudo apt install ros-dashing-navigation2
+      sudo apt install ros-dashing-nav2-bringup
 
 3. Install the Turtlebot 3 packages. Again, for Ubuntu 18, it looks like this: