Assure the description is loaded as string

If this isn't explicitly specified, the description string might be
interpreted as a yaml content, which leads to problems, obviously.

ur_robot_driver/launch/ur_rsp.launch.py

@@ -31,6 +31,7 @@
 
 from launch_ros.actions import Node
 from launch_ros.substitutions import FindPackageShare
+from launch_ros.parameter_descriptions import ParameterValue
 
 from launch import LaunchDescription
 from launch.actions import DeclareLaunchArgument
@@ -185,7 +186,9 @@ def generate_launch_description():
             " ",
         ]
     )
-    robot_description = {"robot_description": robot_description_content}
+    robot_description = {
+        "robot_description": ParameterValue(robot_description_content, value_type=str)
+    }
 
     declared_arguments = []
     # UR specific arguments