Fixed documentation typo

Author: NamanAgarwal0905

README.md

@@ -71,7 +71,7 @@ commands.
 
 *Nightly binaries are available for testing using the
 [tf-nightly](https://pypi.python.org/pypi/tf-nightly) and
-[tf-nightly-cpu](https://pypi.python.org/pypi/tf-nightly-cpu) packages on PyPi.*
+[tf-nightly-cpu](https://pypi.python.org/pypi/tf-nightly-cpu) packages on PyPI.*
 
 #### *Try your first TensorFlow program*