doc: update bors link (#8588)

Author: alexjbest

README.md

@@ -2,7 +2,7 @@
 
 ![](https://github.com/leanprover-community/mathlib4/workflows/continuous%20integration/badge.svg?branch=master)
 ![GitHub CI](https://github.com/leanprover-community/mathlib4/workflows/continuous%20integration/badge.svg?branch=master)
-[![Bors enabled](https://bors.tech/images/badge_small.svg)](https://app.bors.tech/repositories/37904)
+[![Bors enabled](https://bors.tech/images/badge_small.svg)](https://mathlib-bors-ca18eefec4cb.herokuapp.com/repositories/16)
 [![project chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://leanprover.zulipchat.com)
 
 This is a complete port of [mathlib](https://github.com/leanprover-community/mathlib) to [Lean 4](https://leanprover.github.io/).