Update intro_tutorial.ipynb

improve visual of intro tutorial

docs/tutorials/intro_tutorial.ipynb

@@ -25,7 +25,7 @@
     "- Recovery Rate (time infection lasts)\n",
     "- Mobility (distance agent moves)\n",
     "\n",
-    "### The tutorial then proceeds in three parts: \n",
+    "**The tutorial then proceeds in three parts:** \n",
     "- Part 1 Create the Basic Model \n",
     "- Part 2 Add Agent Behaviors and Model Complexity \n",
     "- Part 3 Add Visualizations and Interface\n",