Update README.md

Fixed typo

README.md

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 # THEMIS  
-The Themis convention library contains all ratified conventions and is located <a href="http://forums.ohdsi.org/tags/themis">here.</a>
+The Themis convention library contains all ratified conventions and is located <a href="https://ohdsi.github.io/Themis/index.html">here.</a>  
 
 ## Introduction
 OHDSI Community: Themis makes decisions for the good of the whole community. We must compromise. We can always revisit and modify the convention. Don’t let perfect be the enemy of great. And interoperability between different OMOP CDMs is great!