Update README.md

Update URL for the plugin in Icy

README.md

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 ## Plugins for Image Analysis GUIs
 Parts of Pottslab can be used without Matlab as pure Java plugins
-   - [Icy plugin](http://icy.bioimageanalysis.org/plugin/Potts_Segmentation) - an interactive image segmentation plugin based on Pottslab (written by Vasileios Angelopoulos)
+   - [Icy plugin](http://icy.bioimageanalysis.org/plugin/potts-segmentation/) - an interactive image segmentation plugin based on Pottslab (written by Vasileios Angelopoulos)
    - [ImageJ plugin](Plugins/PottsSegmentationJ_.jar) - an ImageJ frontend for Pottslab (written by Michael Kaul) 
 
 ## References