Correct small coding typo

16_nlp_with_rnns_and_attention.ipynb

@@ -1383,7 +1383,7 @@
    "outputs": [],
    "source": [
     "def string_to_ids(s, chars=POSSIBLE_CHARS):\n",
-    "    return [POSSIBLE_CHARS.index(c) for c in s]"
+    "    return [chars.index(c) for c in s]"
    ]
   },
   {