README: fix pip install link

Merge pull request #4 from broady/patch-1

----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118742515

Author: silviulica

README.md

@@ -178,7 +178,7 @@ we'll assume you use that one.  With a virtual environment active, paste the
 URL into a ``pip install`` shell command, executing something like this:
 
 ```sh
-pip install https://github.com/GoogleCloud/DataflowPythonSDK/va.b.c.tar.gz
+pip install https://github.com/GoogleCloudPlatform/DataflowPythonSDK/vX.Y.Z.tar.gz
 ```
 
 #### Notes on installing with ``setup.py install``