fix heapster reference to master branch

Author: andyxning

README.md

@@ -66,7 +66,7 @@ For example, to run without auth, use the following config:
 ```
 http://APISERVER_IP:APISERVER_PORT?inClusterConfig=false
 ```
-Refer [heapster docs](https://github.com/kubernetes/heapster/blob/1e40b0f4b5eeb3f02e11ee22c2b6fda36b6e6ea1/docs/source-configuration.md#kubernetes) for a complete list of available options.
+Refer [heapster docs](https://github.com/kubernetes/heapster/blob/master/docs/source-configuration.md#kubernetes) for a complete list of available options.
 * `--hostname-override`: A customized node name used for node-problem-detector to update conditions and emit events. node-problem-detector gets node name first from `hostname-override`, then `NODE_NAME` environment variable and finally fall back to `os.Hostname`.
 
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