Update README.md (#235)

Somebody decided to move all of the Hololens documentation and as a result pretty much every existing link on the internet is broken.  Fixing this one.

README.md

@@ -28,7 +28,7 @@ For documentation on the WDP REST endpoints which the project wraps, see the fol
 
 ### HoloLens specific methods
 
-[HoloLens Device Portal API reference](https://developer.microsoft.com/en-us/windows/holographic/device_portal_api_reference)
+[HoloLens Device Portal API reference](https://developer.microsoft.com/en-us/windows/mixed-reality/device_portal_api_reference)
 
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