add lab builtin solutions

Author: fabridamicelli

book/lab_builtin_for_the_win.ipynb

@@ -261,7 +261,7 @@
     "    \"rewards\": some-value,\n",
     "}\n",
     "```\n",
-    "6) Sort the list of dictionaries according to the rewards value. \n",
+    "6) Sort the list of dictionaries according to the rewards value . Hint: Discard missing reward values. \n",
     "7) Repeat the same but in descendant order (larger values first).\n",
     "8) Make a list of the unique \"trial_type\" values that appear in the data.\n",
     "9) Make a dictionary that counts the number of appearances of each \"trial_type\".\n",