Update microstrip.pro

Electrostatics/microstrip.pro

@@ -190,7 +190,7 @@ Formulation {
      product over a domain D. If the test-functions v' are differentiable,
      integration by parts using Green's identity leads to finding v such that
 
-     (epsilon grad v, grad v')_Vol_Ele + (n . (epsilon grad v), v')_Bnd_Vol_Ele = 0
+     (epsilon grad v, grad v')_Vol_Ele - (n . (epsilon grad v), v')_Bnd_Vol_Ele = 0
 
      holds for all v', where Bnd_Vol_Ele is the boundary of Vol_Ele. In our
      microstrip example this surface term vanishes, as there is either no test