fix iteration error (#208)

* fix iteration error

array iteration error: sentences up till max_len of token exceeds should be returned

* split list once instead of twice

split list once instead of twice and take parsed string up till max token length is exceeded

examples/fine-tuned_qa/olympics-1-collect-data.ipynb

@@ -154,7 +154,7 @@
     "        for i, sentence in enumerate(sentences):\n",
     "            ntokens += 1 + count_tokens(sentence)\n",
     "            if ntokens > max_len:\n",
-    "                return \". \".join(sentences[:i][:-1]) + \".\"\n",
+    "                return \". \".join(sentences[:i]) + \".\"\n",
     "\n",
     "    return long_text\n",
     "\n",