tutorial update: made nDom unsigned to remove warnings

tutorials/helmholtz/waveguide/main.cpp

@@ -89,7 +89,7 @@ int main(int argc, char **argv)
 
   double pi = 3.14159265359;
 
-  int nDom = 3;
+  unsigned int nDom = 3;
   gmshDdm.userDefinedParameter(nDom, "nDom");
   double L = 5;
   double l = 1;