Corrected spelling in comments

Resolves bug-report https://bugs.openfoam.org/view.php?id=2844

src/waves/waveModels/Airy/Airy.C

@@ -161,7 +161,7 @@ Foam::tmp<Foam::scalarField> Foam::waveModels::Airy::pressure
 ) const
 {
     // It is a fluke of the formulation that the time derivative of the velocity
-    // potential equals the x-derivative multipled by the celerity. This allows
+    // potential equals the x-derivative multiplied by the celerity. This allows
     // for this shortcut in evaluating the unsteady pressure.
     return celerity()*velocity(t, u, xz)->component(0);
 }