try to fix badge

Author: kleinhenz

README.md

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 # SlurmClusterManager.jl
 
-![Build Status](https://github.com/kleinhenz/SlurmClusterManager.jl/workflows/CI/badge.svg?branch=master)
+![Build Status](https://github.com/kleinhenz/SlurmClusterManager.jl/workflows/CI/badge.svg)
 
 This package provides support for using julia within the Slurm cluster environment.
 The code is adapted from [ClusterManagers.jl](https://github.com/JuliaParallel/ClusterManagers.jl) with some modifications.