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.

Author: Felix Exner

README.md

@@ -114,7 +114,7 @@ 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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