Fix doc links to use github.io (#124)

Author: liyanhui1228

README.rst

@@ -14,7 +14,7 @@ OpenCensus for Python - A stats collection and distributed tracing framework
 
 -  `API Documentation`_
 
-.. _API Documentation: http://opencensus.io/opencensus-python/trace/usage.html
+.. _API Documentation: http://census-instrumentation.github.io/opencensus-python/trace/usage.html
 
 Installation & basic usage
 --------------------------