Make appveyor badge point to correct branch

README.md

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 # PQClean
 
 [![Build Status](https://travis-ci.com/PQClean/PQClean.svg?branch=master)](https://travis-ci.com/PQClean/PQClean)
-[![Build status](https://ci.appveyor.com/api/projects/status/186ky7yb9mlqj3io?svg=true)](https://ci.appveyor.com/project/PQClean/pqclean)
+[![Build status](https://ci.appveyor.com/api/projects/status/186ky7yb9mlqj3io/branch/master?svg=true)](https://ci.appveyor.com/project/PQClean/pqclean/branch/master)
 
 **PQClean**, in short, is an effort to collect **clean** implementations of the post-quantum
 schemes that are in the