[examples] Specify CollisionResponse (#17)

According to sofa-framework/sofa#5604

Author: hugtalbot

examples/ParticlesRepulsionForceField.scn

@@ -22,7 +22,7 @@
     <CollisionPipeline verbose="0" />
     <BruteForceBroadPhase/>
     <BVHNarrowPhase/>
-    <CollisionResponse name="Response" />
+    <CollisionResponse name="Response" response="PenalityContactForceField"/>
     <NewProximityIntersection alarmDistance="0.002" contactDistance="0.001" />
 
     <Node name="Floor">