Minor in MPI notebooks

notebooks/mpi_collectives.ipynb

@@ -120,6 +120,16 @@
     "In this example the ranks sleep for a random amount of time and then they call barrier. It is guaranteed that the message \"Done!\" will be printed after all processes printed \"I woke up\" since we used a barrier. Try also to comment out the call to `MPI.Barrier`. You will see that the message can be printed in any order."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "c6e78389",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "using MPI"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,