Fix stable docs link in README.md (#249)

Author: jacobusmmsmit

README.md

@@ -1,6 +1,6 @@
 # ReverseDiff
 
-[![Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliadiff.org/ReverseDiff.jl/stable)
+[![Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliadiff.org/ReverseDiff.jl/)
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliadiff.org/ReverseDiff.jl/dev)
 [![Build status](https://github.com/JuliaDiff/ReverseDiff.jl/workflows/CI/badge.svg)](https://github.com/JuliaDiff/ReverseDiff.jl/actions)
 [![codecov.io](https://codecov.io/github/JuliaDiff/ReverseDiff.jl/coverage.svg?branch=master)](https://codecov.io/github/JuliaDiff/ReverseDiff.jl?branch=master)