[lesson3_answers] Fix title typo from L2 to L3.

Author: mmmarinho

basic_lessons/lesson3_exercise_answers.ipynb

@@ -4,7 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# L2 Exercise Answers\n",
+    "# L3 Exercise Answers\n",
     "\n",
     "*License: CC-BY-NC-SA 4.0*\n",
     "\n",