Update README.md (#5)

Updating README to include correct file name.
diff --git a/README.md b/README.md
index 1641438..7cb552d 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@
 
 Finally, here is the sample message posted on Slack showing list of delayed pull requrests from OpenWhisk repo. OpenWhisk project developers at IBM are using this bot to receive such notifications.
 
-![Pull Requests Notification](docs/images/slack-pr-review-messages.jpg "Delayed Pull Requests Notification")
+![Pull Requests Notification](docs/images/slack-pr-review-messages.png "Delayed Pull Requests Notification")
 
 ### Bringing it all together