fix README URL

Author: imaginator

README.md

@@ -2,7 +2,7 @@
 
 Simplicity is a minimal, deterministic, functional language built for Bitcoin’s trust model. It offers elegance over complexity, enabling powerful smart contracts without relying on a growing list of ad hoc opcodes.
 
-The site is built on `Material for MkDocs` 
+The site is built on [Material for MkDocs](https://squidfunk.github.io/mkdocs-material/)
 
 Building