[docs] update GitHub Actions badge (#3446)

README.md

@@ -3,7 +3,7 @@
 Light Gradient Boosting Machine
 ===============================
 
-[![GitHub Actions Build Status](https://github.com/microsoft/LightGBM/workflows/GitHub%20Actions/badge.svg?branch=master)](https://github.com/microsoft/LightGBM/actions)
+[![GitHub Actions Build Status](https://github.com/microsoft/LightGBM/workflows/R%20GitHub%20Actions/badge.svg?branch=master)](https://github.com/microsoft/LightGBM/actions)
 [![Azure Pipelines Build Status](https://lightgbm-ci.visualstudio.com/lightgbm-ci/_apis/build/status/Microsoft.LightGBM?branchName=master)](https://lightgbm-ci.visualstudio.com/lightgbm-ci/_build/latest?definitionId=1)
 [![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/1ys5ot401m0fep6l/branch/master?svg=true)](https://ci.appveyor.com/project/guolinke/lightgbm/branch/master)
 [![Travis Build Status](https://travis-ci.org/microsoft/LightGBM.svg?branch=master)](https://travis-ci.org/microsoft/LightGBM)