revise github actions badge (#4079)

revise urls so that badge reflects status of the CI workflow for the stable
branch and links specifically to run results for the same

Author: michaelsbradleyjr

README.md

@@ -1,6 +1,6 @@
 # Nimbus Eth2 (Beacon Chain)
 
-[![Github Actions CI](https://github.com/status-im/nimbus-eth2/actions/workflows/ci.yml/badge.svg)](https://github.com/status-im/nimbus-eth2/actions/workflows/ci.yml)
+[![Github Actions CI](https://github.com/status-im/nimbus-eth2/actions/workflows/ci.yml/badge.svg?branch=stable)](https://github.com/status-im/nimbus-eth2/actions/workflows/ci.yml?query=branch%3Astable)
 [![License: Apache](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 ![Stability: experimental](https://img.shields.io/badge/stability-experimental-orange.svg)