Fix minor typo and remove duplicated def of n_active (#20)

notebooks/01-forward-dc-resisitivity-2d.ipynb

@@ -619,16 +619,6 @@
     "```"
    ]
   },
-  {
-   "cell_type": "code",
-   "execution_count": 19,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "# number of active cells\n",
-    "n_active = np.sum(active_cells)"
-   ]
-  },
   {
    "cell_type": "code",
    "execution_count": 20,
@@ -686,7 +676,7 @@
     "\n",
     "### <span style=\"color:darkgreen\">Exercise (beginner):</span>\n",
     "\n",
-    "Use [dc.simulation_2d.Simulation2DNodel](https://docs.simpeg.xyz/content/api/generated/SimPEG.electromagnetics.static.resistivity.Simulation2DNodal.html#SimPEG.electromagnetics.static.resistivity.Simulation2DNodal) to define the simulation. We instantiate the simulation by setting the following:\n",
+    "Use [dc.simulation_2d.Simulation2DNodal](https://docs.simpeg.xyz/content/api/generated/SimPEG.electromagnetics.static.resistivity.Simulation2DNodal.html#SimPEG.electromagnetics.static.resistivity.Simulation2DNodal) to define the simulation. We instantiate the simulation by setting the following:\n",
     "\n",
     "* **mesh** (the mesh as an input argument)\n",
     "* **survey=survey** (set the survey as a keyword argument)\n",