message: Fix Windows build status badge to show master branch
  Used to show the latest build, which can come from PR
diff: | Linux | Windows |
  |-------|---------|
  -| [![Build Status](https://travis-ci.org/onnx/onnx.svg?branch=master)](https://travis-ci.org/onnx/onnx) | [![Build status](https://ci.appveyor.com/api/projects/status/lm50cevk2hmrll98?svg=true)](https://ci.appveyor.com/project/onnx/onnx) |
  +| [![Build Status](https://travis-ci.org/onnx/onnx.svg?branch=master)](https://travis-ci.org/onnx/onnx) | [![Build status](https://ci.appveyor.com/api/projects/status/lm50cevk2hmrll98/branch/master?svg=true)](https://ci.appveyor.com/project/onnx/onnx) |
  [Open Neural Network Exchange (ONNX)](http://onnx.ai) is the first step toward an open ecosystem that empowers AI developers
  to choose the right tools as their project evolves. ONNX provides an open source format for AI models.
  