How to add RaySensor plugin to the world file?

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According to modification of this tutorial I managed to create a plugin for a ray sensor. I also added the line

<plugin name="RayPlugin" filename="build/libRayPlugin.so"/>

to the .world file directly bellow

<sensor name='my_sensor' type='ray'>

just like in the tutorial. However, when I open the world in Gazebo, the plugin doesn't appear there. The path to the plugin is correct, as I am able to use model plugins. I'm using Gazebo 9 on Ubuntu 18.4.1.