Update README.md

Author: emmahodcroft

README.md

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 # Pathoplexus Example Data
 
-Example files for users to try out on the Pathoplexus [demo instance](). _(Please **do not** upload these to the main website!)_
+Example files for users to try out on the Pathoplexus [demo instance](https://demo.pathoplexus.org/). _(Please **do not** upload these to the main website!)_
 
 Currently holds files for West Nile virus, Ebola Zaire, and CCHF.