Fix the transformer link

official/README-TPU.md

@@ -5,7 +5,7 @@
 *   [bert](nlp/bert): A powerful pre-trained language representation model:
     BERT, which stands for Bidirectional Encoder Representations from
     Transformers.
-*   [transformer](transformer): A transformer model to translate the WMT
+*   [transformer](nlp/transformer): A transformer model to translate the WMT
     English to German dataset.
 
 ## Computer Vision