PairSpringSliderDashpot: Correct sphere interaction calculation for non-equal diameters

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1315

src/lagrangian/intermediate/submodels/Kinematic/CollisionModel/PairCollision/PairModel/PairSpringSliderDashpot/PairSpringSliderDashpot.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2014 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -149,7 +149,7 @@ public:
                     (-rAB + rA - rB)
                    *(-rAB - rA + rB)
                    *(-rAB + rA + rB)
-                   *( rAB + rA + rA)
+                   *( rAB + rA + rB)
                 );
         }