update hello_world

exercises/MPI_examples/hello_world/hello_world.c

@@ -2,10 +2,8 @@
 #include <stdio.h>
 
 int main(int argc, char **argv) {
-  // Initialize the MPI environment. The two arguments to MPI Init are not
-  // currently used by MPI implementations, but are there in case future
-  // implementations might need the arguments.
-  MPI_Init(NULL, NULL);
+  // Initialize the MPI environment.
+  MPI_Init(&argc, &argv);
 
   // Get the number of processes
   int num_processes;