fixed hello world install instructions (#3)

README.md

@@ -10,12 +10,11 @@ newer, which includes NPM 6.4.1 or newer, from nodejs.org
 
 ## How to Build and Run the Hello World sample
 From command prompt:
-* `git clone http://github.com/microsoft/mixed-reality-extension-sdk`
-* `cd mixed-reality-extension-sdk\node`
+* `git clone http://github.com/microsoft/mixed-reality-extension-sdk-samples`
+* `cd mixed-reality-extension-sdk-samples\samples\hello-world`
 * `npm install` This will install all dependent packages. (and will do very
 little if there are no changes)
 * `npm run build` This should not report any errors.
-* `cd packages/samples/hello-world`
 * `npm start` This should print "INF: Multi-peer Adapter listening on..."
 
 In AltspaceVR