update docs link (#530)

Author: jverzani

README.md

@@ -1,6 +1,6 @@
 [![Build status](https://github.com/JuliaPy/SymPy.jl/workflows/CI/badge.svg)](https://github.com/JuliaPy/SymPy.jl/actions)
  
-[![](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliahub.com/docs/SymPy)
+[![](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliapy.github.io/SymPy.jl/dev/)