Upgrade the wheel URL in requirements_docs.txt (#1721)

Author: st-pasha

requirements_docs.txt

@@ -1,3 +1,3 @@
 sphinx>=1.8
 nbsphinx>=0.3.5
-https://s3.amazonaws.com/artifacts.h2o.ai/releases/ai/h2o/pydatatable/0.7.0.dev492/x86_64-centos7/datatable-0.7.0.dev492-cp35-cp35m-linux_x86_64.whl
+https://s3.amazonaws.com/artifacts.h2o.ai/releases/ai/h2o/pydatatable/0.8.0.dev498/x86_64-centos7/datatable-0.8.0.dev498-cp37-cp37m-linux_x86_64.whl