Fix missing comma in comment.

config/checksrc/openmpi_dup.c

@@ -100,7 +100,7 @@ int main(void) {
     {
       MPI_Datatype sends[2];
 
-      // by definition of MPI_Type_dup no MPI_Type_commit is required
+      // by definition of MPI_Type_dup, no MPI_Type_commit is required
       MPI_Type_dup(MPI_INT, &sends[0]);
 
       {