Update README.md to point to renamed mojo/tools/mojo_demo.py.

BUG=
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/841863002

README.md

diff --git a/README.md b/README.md
index 25c6ad7..dd2e004 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,7 @@
 
 Run a demo:
 ```
-mojo/tools/mojo_demo.sh --browser
+mojo/tools/mojo_demo.py --browser
 ```
 
 Run the tests: