Fixes for public imdb tutorials.

PiperOrigin-RevId: 245428010

Author: MarkDaoust

site/en/r2/tutorials/keras/basic_text_classification.ipynb

@@ -125,7 +125,7 @@
       "source": [
         "from __future__ import absolute_import, division, print_function, unicode_literals\n",
         "\n",
-        "!pip install tensorflow==2.0.0-alpha0\n",
+        "!pip install tf-nightly-2.0-preview\n",
         "import tensorflow as tf\n",
         "from tensorflow import keras\n",
         "\n",