update readme for Unity version (2021 for now)

README.md

@@ -26,7 +26,7 @@ or clone the repos updating the submodules directly:
 
     git clone --recurse-submodules https://labs.etsi.org/rep/arf/world-storage-api-helpers/unity-world-storage-editor.git
 
-Once it's done, you can open your project from the Unity Hub, the project was developed with Unity 2022.3
+Once it's done, you can open your project from the Unity Hub, the project was developed with Unity 2021.3
 
 ## Test Scene