Explicitly set branch name in Codecov badge url

Author: bowenszhu

README.md

@@ -1,7 +1,7 @@
 # ModelOrderReduction.jl
 
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 [![SciML Code Style](https://img.shields.io/static/v1?label=code%20style&message=SciML&color=9558b2&labelColor=389826)](https://github.com/SciML/SciMLStyle)
 
 ModelOrderReduction.jl is a package for automatically reducing the computational complexity