Update README.md

Author: catqaq

README.md

@@ -7,7 +7,7 @@
 - Support pretrained word embedding.([word2vec](https://github.com/mmihaltz/word2vec-GoogleNews-vectors))
 
 ## torchtext
-- This package can provide an elegant way to build vocabulary([yahoo](https://s3.amazonaws.com/fast-ai-nlp/yahoo_answers_csv.tgz)). 
+- This package can provide an elegant way to build vocabulary([yahoo](https://torchtext.readthedocs.io/en/latest/index.html#)). 
 ```
 TEXT.build_vocab(dataset, vectors)
 ```