update chapter title

ch03/01_main-chapter-code/ch03.ipynb

@@ -5,7 +5,7 @@
    "id": "8bfa70ec-5c4c-40e8-b923-16f8167e3181",
    "metadata": {},
    "source": [
-    "# Chapter 3: Understanding Attention Mechanisms"
+    "# Chapter 3: Coding Attention Mechanisms"
    ]
   },
   {