Update a description to get it meaningful

This lack was highlighted in user tests.

Author: Bouh

extensions/reviewed/PhysicsCar.json

@@ -2260,7 +2260,7 @@
           "type": "Number",
           "unit": "PixelAcceleration",
           "label": "Acceleration",
-          "description": "",
+          "description": "Object speed",
           "group": "Speed",
           "extraInformation": [],
           "name": "Acceleration"
@@ -2424,4 +2424,4 @@
     }
   ],
   "eventsBasedObjects": []
-}
+}