Update README.md to make it more clear

exercises/exercise-properties/README.md

@@ -67,7 +67,7 @@ SET_TYPE_PROP(TwoPIncompressible, LocalResidual,
               MyLocalResidual<TypeTag>);
 ```
 
-You want to simplify the original local residual by using the assumption that incompressible fluid phases are present. As a consequence, one can balance phase volume instead of phase mass.
+You want to simplify the original local residual by using the assumption that only incompressible fluid phases are present. As a consequence, one can balance phase volume instead of phase mass.
 
 * Eliminate the density from the `computeStorage` and the `computeFlux` methods.