Update datatable link in requirements_doc.txt

Author: st-pasha

requirements_docs.txt

@@ -1,4 +1,4 @@
 docutils>=0.14
 sphinx>=1.8
 nbsphinx>=0.3.5
-https://s3.amazonaws.com/artifacts.h2o.ai/releases/ai/h2o/pydatatable/0.8.0.dev551/x86_64-centos7/datatable-0.8.0.dev551-cp37-cp37m-linux_x86_64.whl
+https://s3.amazonaws.com/artifacts.h2o.ai/releases/ai/h2o/pydatatable/0.9.0/x86_64-centos7/datatable-0.9.0-cp37-cp37m-linux_x86_64.whl