Change install instructions to install metapackage

Before we were telling users to install the driver package only. However,
it probably makes sense suggesting to install the ur meta package.

(cherry picked from commit d2599ec)

Author: fmauch

README.md

@@ -134,7 +134,7 @@ that will behave almost exactly like the real robot.
       branches.
 2. Install the driver using
    ```
-   sudo apt-get install ros-${ROS_DISTRO}-ur-robot-driver
+   sudo apt-get install ros-${ROS_DISTRO}-ur
    ```
 
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