updated comment.

Source/Particles/Collision/BinaryCollision/Coulomb/ElasticCollisionPerez.H

@@ -116,7 +116,7 @@ void ElasticCollisionPerez (
 #endif
 
           // compute the effective density used for s12 in UpdateMomentumPerezElastic()
-          // The effective density used for s12 is defined such that average value of s12
+          // The effective density used for s12 is defined such that the average value of s12
           // after many scattering cycles is the same as that for the full NxN pairing method.
           // The approach here is similar to that in JCP 413 (2020) by Higginson et al., but
           // does not use a unique duplication factor for each macro-particle pair.